[SPARK-30170][SQL][MLLIB][TESTS] Eliminate compilation warnings: part 1

### What changes were proposed in this pull request?
- Replace `Seq[String]` by `Seq[_]` in `StopWordsRemoverSuite` because `String` type is unchecked due erasure.
- Throw an exception for default case in `MLTest.checkNominalOnDF` because we don't expect other attribute types currently.
- Explicitly cast float to double in `BigDecimal(y)`. This is what the `apply()` method does for `float`s.
- Replace deprecated `verifyZeroInteractions` by `verifyNoInteractions`.
- Equivalent replacement of `\0` by `\u0000` in `CSVExprUtilsSuite`
- Import `scala.language.implicitConversions` in `CollectionExpressionsSuite`, `HashExpressionsSuite` and in `ExpressionParserSuite`.

### Why are the changes needed?
The changes fix compiler warnings showed in the JIRA ticket https://issues.apache.org/jira/browse/SPARK-30170 . Eliminating the warning highlights other warnings which could take more attention to real problems.

### Does this PR introduce any user-facing change?
No

### How was this patch tested?
By existing test suites `StopWordsRemoverSuite`, `AnalysisExternalCatalogSuite`, `CSVExprUtilsSuite`, `CollectionExpressionsSuite`, `HashExpressionsSuite`, `ExpressionParserSuite` and sub-tests of `MLTest`.

Closes #26799 from MaxGekk/eliminate-warning-2.

Authored-by: Maxim Gekk <max.gekk@gmail.com>
Signed-off-by: Sean Owen <srowen@gmail.com>

mllib/src/test/scala/org/apache/spark/ml/feature/StopWordsRemoverSuite.scala

@@ -242,7 +242,7 @@ class StopWordsRemoverSuite extends MLTest with DefaultReadWriteTest {
     remover.transform(df)
       .select("filtered1", "expected1", "filtered2", "expected2")
       .collect().foreach {
-        case Row(r1: Seq[String], e1: Seq[String], r2: Seq[String], e2: Seq[String]) =>
+        case Row(r1: Seq[_], e1: Seq[_], r2: Seq[_], e2: Seq[_]) =>
           assert(r1 === e1,
             s"The result value is not correct after bucketing. Expected $e1 but found $r1")
           assert(r2 === e2,
@@ -268,7 +268,7 @@ class StopWordsRemoverSuite extends MLTest with DefaultReadWriteTest {
     remover.transform(df)
       .select("filtered1", "expected1", "filtered2", "expected2")
       .collect().foreach {
-        case Row(r1: Seq[String], e1: Seq[String], r2: Seq[String], e2: Seq[String]) =>
+        case Row(r1: Seq[_], e1: Seq[_], r2: Seq[_], e2: Seq[_]) =>
           assert(r1 === e1,
             s"The result value is not correct after bucketing. Expected $e1 but found $r1")
           assert(r2 === e2,

mllib/src/test/scala/org/apache/spark/ml/util/MLTest.scala

@@ -88,6 +88,8 @@ trait MLTest extends StreamTest with TempDirectory { self: Suite =>
     val n = Attribute.fromStructField(dataframe.schema(colName)) match {
       case binAttr: BinaryAttribute => Some(2)
       case nomAttr: NominalAttribute => nomAttr.getNumValues
+      case unknown =>
+        throw new IllegalArgumentException(s"Attribute type: ${unknown.getClass.getName}")
     }
     assert(n.isDefined && n.get === numValues,
       s"the number of values obtained from schema should be $numValues, but got $n")

sql/catalyst/src/main/scala/org/apache/spark/sql/types/FloatType.scala

@@ -78,8 +78,12 @@ case object FloatType extends FloatType {
   }
 
   trait FloatAsIfIntegral extends FloatIsConflicted with Integral[Float] {
-    def quot(x: Float, y: Float): Float = (BigDecimal(x) quot BigDecimal(y)).floatValue
-    def rem(x: Float, y: Float): Float = (BigDecimal(x) remainder BigDecimal(y)).floatValue
+    def quot(x: Float, y: Float): Float = {
+      (BigDecimal(x.toDouble) quot BigDecimal(y.toDouble)).floatValue
+    }
+    def rem(x: Float, y: Float): Float = {
+      (BigDecimal(x.toDouble) remainder BigDecimal(y.toDouble)).floatValue
+    }
   }
 
   object FloatAsIfIntegral extends FloatAsIfIntegral {

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/analysis/AnalysisExternalCatalogSuite.scala

@@ -59,7 +59,7 @@ class AnalysisExternalCatalogSuite extends AnalysisTest with Matchers {
         Alias(UnresolvedFunction("sum", Seq(UnresolvedAttribute("a")), isDistinct = false), "s")()
       val plan = Project(Seq(func), testRelation)
       analyzer.execute(plan)
-      verifyZeroInteractions(catalog)
+      verifyNoInteractions(catalog)
     }
   }
 
@@ -73,7 +73,7 @@ class AnalysisExternalCatalogSuite extends AnalysisTest with Matchers {
         ignoreIfExists = false)
       reset(externCatalog)
       catalog.functionExists(FunctionIdentifier("sum"))
-      verifyZeroInteractions(externCatalog)
+      verifyNoInteractions(externCatalog)
     }
   }
 

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/csv/CSVExprUtilsSuite.scala

@@ -78,7 +78,7 @@ class CSVExprUtilsSuite extends SparkFunSuite {
     // null character, expressed in Unicode literal syntax
     ("""\u0000""", Some("\u0000"), None),
     // and specified directly
-    ("\0", Some("\u0000"), None)
+    ("\u0000", Some("\u0000"), None)
   )
 
   test("should correctly produce separator strings, or exceptions, from input") {

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/CollectionExpressionsSuite.scala

@@ -20,6 +20,7 @@ package org.apache.spark.sql.catalyst.expressions
 import java.sql.{Date, Timestamp}
 import java.util.TimeZone
 
+import scala.language.implicitConversions
 import scala.util.Random
 
 import org.apache.spark.SparkFunSuite

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/HashExpressionsSuite.scala

@@ -21,6 +21,7 @@ import java.nio.charset.StandardCharsets
 import java.time.{ZoneId, ZoneOffset}
 
 import scala.collection.mutable.ArrayBuffer
+import scala.language.implicitConversions
 
 import org.apache.commons.codec.digest.DigestUtils
 import org.scalatest.exceptions.TestFailedException

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/parser/ExpressionParserSuite.scala

@@ -20,6 +20,8 @@ import java.sql.{Date, Timestamp}
 import java.time.LocalDateTime
 import java.util.concurrent.TimeUnit
 
+import scala.language.implicitConversions
+
 import org.apache.spark.sql.catalyst.FunctionIdentifier
 import org.apache.spark.sql.catalyst.analysis.{UnresolvedAttribute, _}
 import org.apache.spark.sql.catalyst.expressions._