[flink] Fix watermark generation for empty UnboundedSource

[je-ik]

Closes #30050

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[je-ik]

R: @Abacn @JozoVilcek @dmvk

[je-ik]

Run Flink ValidatesRunner

[github-actions]

Stopping reviewer notifications for this pull request: review requested by someone other than the bot, ceding control

[JozoVilcek]

lgtm

[je-ik]

Run Flink ValidatesRunner

[Abacn]

Hi, we have migrated to GitHub Action and the way to run PostCommit has changed due to some limitation of GHA. To run Flink ValidatesRunner PostCommit, add an empty file named .github/trigger_files/beam_PostCommit_Java_ValidatesRunner_Flink.json and attach to your PR, see https://github.com/apache/beam/tree/master/.github/workflows#comment-triggering-support

[je-ik]

Ah, thanks.

[Abacn]

though the Jenkins is still running a few job (with only a couple of workers)

For this PR, the result is in https://ci-beam.apache.org/job/beam_PostCommit_Java_ValidatesRunner_Flink_PR/511/

[Abacn]

testTeardownCalledAfterExceptionInStartBundleStateful is a known flaky test: #19814

[Abacn]

The newly added test passed: https://ge.apache.org/s/i3siknxbqnh5a/tests/task/:runners:flink:1.16:test/details/org.apache.beam.runners.flink.translation.wrappers.streaming.io.source.unbounded.FlinkUnboundedSourceReaderTest/testWatermarkOnEmptySource?top-execution=1