[FLINK-22143][table runtime] fix less lines than requested returned by LimitableBulk…

[iyupeng]

…Format

What is the purpose of the change

• fix less lines than requested returned by LimitableBulkFormat under certain conditions

Brief change log

• check null or not on result of next() before increase numRead

Verifying this change

This change is already covered by existing tests

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

• Dependencies (does it add or upgrade a dependency): (yes / no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (yes / no)
• The serializers: (yes / no / don't know)
• The runtime per-record code paths (performance sensitive): (yes / no / don't know)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn/Mesos, ZooKeeper: (yes / no / don't know)
• The S3 file system connector: (yes / no / don't know)

Documentation

• Does this pull request introduce a new feature? (yes / no)
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)

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Automated Checks

Last check on commit 0e2e25e (Fri May 28 09:01:36 UTC 2021)

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• ❓ 1. The [description] looks good.
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CI report:

• 0e2e25e Azure: SUCCESS

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[KurtYoung]

@iyupeng could you add a test case?

[iyupeng]

@iyupeng could you add a test case?

No problem.

[KurtYoung]

LGTM, thanks for the fix @iyupeng