Add explicit clearSyncTransaction instead of passing in null

When attempting to clear the last syncNextTransaction, the code was
ambiguous and it was unclear what the expectation should be. Instead,
don't allow null when calling syncNextTransaction and don't allow the
syncTransaction to be overwritten. If the caller wants to clear the
syncTransaction before a frame has arrived, they can do so by calling
clearSyncTransaction.

Test: No warning log in sync with no buffer.
Test: BLASTBufferQueueTest
Bug: 272189296
Change-Id: I3a945f5503220225f2147b0331d1fb2f9ea8dc63