Only apply match if there is something to apply

It is possible for a match to only contain a context and not a schema,
or vice versa and in this case, the previously matched values should
continue to be used.  The serializer already handles this case by
setting the index of the match to ~0u if there is not one, so this
change simply has the parser skip these values.

Bug: 70858511
Test: unit tests, including new ones
Change-Id: Ibc65bd8d637e39f3b1ce7bcc2b88189b75173f88