Fix KinesisIO throwing NullPointerException when progress check made before starting reader

[zhoufek]

fixes #23868

This has the KinesisReader return unknown if it cannot determine the backlog size due to the start method not previously being called. The progress check can happen at arbitrary times, so it must be accounted for here.

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[johnjcasey]

Is it valid to try and get the backlog without having started the reader? Whats the actual scenario here?

The fix looks fine

[zhoufek]

Part of the stack trace of the issue is:

...
Caused by: java.lang.NullPointerException
        at org.apache.beam.sdk.io.kinesis.KinesisReader.getSplitBacklogBytes(KinesisReader.java:172)
        at org.apache.beam.sdk.io.Read$UnboundedSourceAsSDFWrapperFn$UnboundedSourceAsSDFRestrictionTracker.getProgress(Read.java:983)
        at org.apache.beam.sdk.transforms.reflect.ByteBuddyDoFnInvokerFactory$DefaultGetSize.invokeGetSize(ByteBuddyDoFnInvokerFactory.java:431)
...

From talking with Cham, it sounds like getProgress can be called at arbitrary times, including before reading starts, and in that case, we should just report no progress.

[johnjcasey]

makes sense to me