[SPARK-32516][SQL][FOLLOWUP] Remove unnecessary check if path string is empty for DataFrameWriter.save(), DataStreamReader.load() and DataStreamWriter.start()

[imback82]

What changes were proposed in this pull request?

This PR is a follow up to #29543 (comment), which correctly points out that the check for the empty string is not necessary.

Why are the changes needed?

The unnecessary check actually could cause more confusion.

For example,

scala> Seq(1).toDF.write.option("path", "/tmp/path1").parquet("")
java.lang.IllegalArgumentException: Can not create a Path from an empty string
  at org.apache.hadoop.fs.Path.checkPathArg(Path.java:168)

even when path option is available. This PR addresses to fix this confusion.

Does this PR introduce any user-facing change?

Yes, now the above example prints the consistent exception message whether the path parameter value is empty or not.

scala> Seq(1).toDF.write.option("path", "/tmp/path1").parquet("")
org.apache.spark.sql.AnalysisException: There is a 'path' option set and save() is called with a path parameter. Either remove the path option, or call save() without the parameter. To ignore this check, set 'spark.sql.legacy.pathOptionBehavior.enabled' to 'true'.;
  at org.apache.spark.sql.DataFrameWriter.save(DataFrameWriter.scala:290)
  at org.apache.spark.sql.DataFrameWriter.parquet(DataFrameWriter.scala:856)
  ... 47 elided

How was this patch tested?

Added unit tests.

[imback82]

This was testing an incorrect API, so I am fixing it here.

[SparkQA]

Test build #128464 has finished for PR 29697 at commit d2c7756.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[cloud-fan]

@imback82 thanks for the followup! can you address this comment as well? https://github.com/apache/spark/pull/29543/files#r485994670 We can use withTempPaths to generate fresh temp paths.

[cloud-fan]

that comment is not an issue, I'm merging it to master, thanks!