perf script: Skip dummy event attr check

Hongtao Yu reported problem when displaying uregs in perf script
for system wide perf.data:

  # perf script -F uregs | head -10
  Samples for 'dummy:HG' event do not have UREGS attribute set. Cannot print 'uregs' field.

The problem is the extra dummy event added for system wide,
which does not have proper sample_type setup.

Skipping attr check completely for dummy event as suggested
by Namhyung, because it does not have any samples anyway.

Reported-by: Hongtao Yu <hoy@fb.com>
Suggested-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Acked-by: Ian Rogers <irogers@google.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20220831124041.219925-1-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>