[hotfix] [table] Fix IndexOutOfBoundsException in DataStreamSortRule.

[fhueske]

What is the purpose of the change

Fixes an IndexOutOfBoundsException in DataStreamSortRule if a plan contains a sort operator without sorting fields (just LIMIT / FETCH).

Brief change log

• Check number of sort fields before checking type of first field.

Verifying this change

Not covered by tests. ORDER BY without sorting fields are not supported cannot be translated. This change just prevents an exception but does not enable the feature.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Yarn/Mesos, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? n/a

[twalthr]

Thanks for the PR @fhueske. I will merge this.