tracepoints: Cache an empty file for failures

tracepoints: Cache an empty file for failures

When a tracepoint header doesn't compile for any reason, we'd like not
to keep trying on subsequent runs.  This patch restores previous
behavior that would save an empty file in the cache, signaling that
we've tried this header before and can't get anything from it.

* tapsets.cxx (tracepoint_builder::get_tracequery_modules): When reading
  from the cache, note and skip empty files.  When saving to the cache,
  copy /dev/null for those that failed to compile.