[SPARK-34926][SQL] PartitioningUtils.getPathFragment() should respect partition value is null

### What changes were proposed in this pull request?

When we insert data into a partition table partition with empty DataFrame. We will call `PartitioningUtils.getPathFragment()`
then to update this partition's metadata too.
When we insert to a partition when partition value is `null`, it will throw exception like
```
[info]   java.lang.NullPointerException:
[info]   at scala.collection.immutable.StringOps$.length$extension(StringOps.scala:51)
[info]   at scala.collection.immutable.StringOps.length(StringOps.scala:51)
[info]   at scala.collection.IndexedSeqOptimized.foreach(IndexedSeqOptimized.scala:35)
[info]   at scala.collection.IndexedSeqOptimized.foreach$(IndexedSeqOptimized.scala:33)
[info]   at scala.collection.immutable.StringOps.foreach(StringOps.scala:33)
[info]   at org.apache.spark.sql.catalyst.catalog.ExternalCatalogUtils$.escapePathName(ExternalCatalogUtils.scala:69)
[info]   at org.apache.spark.sql.catalyst.catalog.ExternalCatalogUtils$.getPartitionValueString(ExternalCatalogUtils.scala:126)
[info]   at org.apache.spark.sql.execution.datasources.PartitioningUtils$.$anonfun$getPathFragment$1(PartitioningUtils.scala:354)
[info]   at scala.collection.TraversableLike.$anonfun$map$1(TraversableLike.scala:238)
[info]   at scala.collection.Iterator.foreach(Iterator.scala:941)
[info]   at scala.collection.Iterator.foreach$(Iterator.scala:941)
[info]   at scala.collection.AbstractIterator.foreach(Iterator.scala:1429)
[info]   at scala.collection.IterableLike.foreach(IterableLike.scala:74)
[info]   at scala.collection.IterableLike.foreach$(IterableLike.scala:73)
```
`PartitioningUtils.getPathFragment()`  should support `null` value too

### Why are the changes needed?
Fix bug

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

### How was this patch tested?
Added UT

Closes #32018 from AngersZhuuuu/SPARK-34926.

Authored-by: Angerszhuuuu <angers.zhu@gmail.com>
Signed-off-by: Max Gekk <max.gekk@gmail.com>

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/catalog/ExternalCatalogUtils.scala

@@ -119,12 +119,16 @@ object ExternalCatalogUtils {
     }
   }
 
-  def getPartitionPathString(col: String, value: String): String = {
-    val partitionString = if (value == null || value.isEmpty) {
+  def getPartitionValueString(value: String): String = {
+    if (value == null || value.isEmpty) {
       DEFAULT_PARTITION_NAME
     } else {
       escapePathName(value)
     }
+  }
+
+  def getPartitionPathString(col: String, value: String): String = {
+    val partitionString = getPartitionValueString(value)
     escapePathName(col) + "=" + partitionString
   }
 

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/PartitioningUtils.scala

@@ -32,6 +32,7 @@ import org.apache.hadoop.fs.Path
 import org.apache.spark.sql.catalyst.InternalRow
 import org.apache.spark.sql.catalyst.analysis.TypeCoercion
 import org.apache.spark.sql.catalyst.catalog.CatalogTypes.TablePartitionSpec
+import org.apache.spark.sql.catalyst.catalog.ExternalCatalogUtils.getPartitionValueString
 import org.apache.spark.sql.catalyst.expressions.{Attribute, Cast, Literal}
 import org.apache.spark.sql.catalyst.util.{CaseInsensitiveMap, DateFormatter, DateTimeUtils, TimestampFormatter}
 import org.apache.spark.sql.errors.{QueryCompilationErrors, QueryExecutionErrors}
@@ -350,7 +351,7 @@ object PartitioningUtils {
    */
   def getPathFragment(spec: TablePartitionSpec, partitionSchema: StructType): String = {
     partitionSchema.map { field =>
-      escapePathName(field.name) + "=" + escapePathName(spec(field.name))
+      escapePathName(field.name) + "=" + getPartitionValueString(spec(field.name))
     }.mkString("/")
   }
 

sql/core/src/test/scala/org/apache/spark/sql/sources/InsertSuite.scala

@@ -933,6 +933,23 @@ class InsertSuite extends DataSourceTest with SharedSparkSession {
       assert(msg.contains("cannot resolve 'c3' given input columns"))
     }
   }
+
+  test("SPARK-34926: PartitioningUtils.getPathFragment() should respect partition value is null") {
+    withTable("t1", "t2") {
+      sql("CREATE TABLE t1(id INT) USING PARQUET")
+      sql(
+        """
+          |CREATE TABLE t2 (c1 INT, part STRING)
+          |  USING parquet
+          |PARTITIONED BY (part)
+          |""".stripMargin)
+      sql(
+        """
+          |INSERT INTO TABLE t2 PARTITION (part = null)
+          |SELECT * FROM t1 where 1=0""".stripMargin)
+      checkAnswer(spark.table("t2"), Nil)
+    }
+  }
 }
 
 class FileExistingTestFileSystem extends RawLocalFileSystem {