[Swift] Make ParserATNSimulator.precedenceTransition return a nullable value. #2408
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This was always clearly a possibility, looking at the body of the method.
The newly-enabled performance tests expose this bug (and I don't know how
we've gotten away with it otherwise for so long).
The Java runtime also returns null at this point.
This fixes the test failures currently seen in PR #2405 and #2407.