Cache failed tracepoint headers too

Cache failed tracepoint headers too

Rather than re-attempting a tracepoint header that fails to compile, we
now save an empty file into the cache to indicate "nothing to see here."
See also PR10424.

* tapsets.cxx (tracepoint_builder::get_tracequery_module): Use /dev/null
  as the empty result file if make_tracequery fails.
  (tracepoint_builder::init_dw): Ignore empty tracequery modules.