perf list: Do not print 'Metric Groups:' unnecessarily

perf list: Do not print 'Metric Groups:' unnecessarily

It was printed unconditionally even if nothing is printed.
Check if the output list empty when filter is given.

Before:
  $ ./perf list duration

  List of pre-defined events (to be used in -e):

    duration_time                                      [Tool event]

  Metric Groups:

After:
  $ ./perf list duration

  List of pre-defined events (to be used in -e):

    duration_time                                      [Tool event]

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lore.kernel.org/lkml/20200909055849.469612-2-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>