[BP-2.0][FLINK-37335][state/forst] Handle the first full checkpoint for ForSt IncrementalSnapshotStrategy.

[AlexYinHan]

What is the purpose of the change

Fix the case when ForSt encounters the first full checkpoint with IncrementalCheckpoint is enabled.

Brief change log

• ForStIncrementalSnapshotStrategy skips throwing an exception if it encounters FORWARD sharingFilesStrategy but there are no previous snapshots to reuse.

Verifying this change

This change added tests and can be verified as follows:

• ForStIncrementalSnapshotStrategyTest#testIncrementalCheckpointWithFirstBeingFull

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, Kubernetes/Yarn, ZooKeeper: (yes)
• The S3 file system connector: (no)

Documentation

• Does this pull request introduce a new feature? (no)

[flinkbot]

CI report:

• b0f3d91 Azure: SUCCESS

Bot commands

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• @flinkbot run azure re-run the last Azure build

[Zakelly]

LGTM