[Fix][Zeta] Fix NPE in completePendingCheckpoint (#10655)

[hyboll]

Purpose of this pull request

Fix avoid NPE in completePendingCheckpoint when notifyCompleted clears pendingCheckpoints

Does this PR introduce any user-facing change?

No

How was this patch tested?

Test case is added in the CheckpointCoordinatorTest.testCompletePendingCheckpointShouldNotThrowNPEWhenNotifyCompletedClearsPendingMap

Check list

• If any new Jar binary package adding in your PR, please add License Notice according
  New License Guide
• If necessary, please update the documentation to describe the new feature. https://github.com/apache/seatunnel/tree/dev/docs
• If necessary, please update incompatible-changes.md to describe the incompatibility caused by this PR.
• If you are contributing the connector code, please check that the following files are updated:
  1. Update plugin-mapping.properties and add new connector information in it
  2. Update the pom file of seatunnel-dist
  3. Add ci label in label-scope-conf
  4. Add e2e testcase in seatunnel-e2e
  5. Update connector plugin_config