[hotfix] Avoid invalid thread for state reader

[beliefer]

What is the purpose of the change

This PR aims to avoid invalid thread for state reader.
Sometimes, the inputGates is an empty array. We can avoid creating thread for it.

Brief change log

Avoid invalid thread for state reader.

Verifying this change

This change is already covered by existing tests, such as (StreamTaskTest).

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: (no)
• The S3 file system connector: (no)

Documentation

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

[flinkbot]

CI report:

• f14a1a1 Azure: SUCCESS

Bot commands

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[davidradl]

nit: prior to the fix there was an Exception now we issue the return. I think there should be a test for this, as there is a change in behaviour.

[beliefer]

There are many test cases trigger this case, such as testFailingAsyncCheckpointRunnable, testStateBackendLoadingAndClosing, testHandleAsyncExceptionDuringRestoring, testProcessWithUnAvailableOutput, testProcessWithRaceInDataAvailability, testEmptySubtaskStateLeadsToStatelessAcknowledgment, testStateBackendClosingOnFailure, testExecuteMailboxActionsAfterLeavingInputProcessorMailboxLoop, testUncaughtExceptionInAsynchronousCheckpointingOperation, testDecliningCheckpointStreamOperator, testProcessWithUnAvailableInput, testAsyncCheckpointingConcurrentCloseAfterAcknowledge, testAsyncCheckpointingConcurrentCloseBeforeAcknowledge, testThreadInvariants, testTaskAvoidHangingAfterSnapshotStateThrownException, testMailboxMetricsScheduling, testRestorePerformedOnlyOnce, testRestorePerformedFromInvoke, testEarlyCanceling, testProcessWithAvailableOutput, testQuiesceOfMailboxRightBeforeSubmittingActionViaTimerService in StreamTaskTest

[beliefer]

ping @zhijiangW @rkhachatryan @AHeise cc @1996fanrui @davidradl @GOODBOY008