NIFI-8102 TailFile can skip characters if tailing a file on NFS mount

[adenes]

• Force the TailFile processor to recreate and reposition the reader
  by setting it to null in case of a NulCharacterEncounteredException
• Updated the TestTailFile.testNULContent() to not initialize the
  processor when calling the second run()

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[markap14]

We can't just abandon the existing reader here, as we need to make sure that it gets closed. Otherwise we'd be leaking a file handle. But I don't think we really want to close and abandon the reader we already have anyway. Since we know where we want the reader to be positioned, we should be able to just use reader.position(positionHolder.get()); no?

[adenes]

Thanks @markap14 , agree, updated the pr

[markap14]

Thanks @adenes . Changes look good to me. Will wait for the Github Actions to complete and assuming that all is green there i'll go ahead and merge.

[markap14]

Looks like all checked out good, +1 merged to main. Thanks @adenes !