### 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>
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