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>

tools/perf/util/metricgroup.c

@@ -535,10 +535,12 @@ void metricgroup__print(bool metrics, bool metricgroups, char *filter,
 		}
 	}
 
-	if (metricgroups && !raw)
-		printf("\nMetric Groups:\n\n");
-	else if (metrics && !raw)
-		printf("\nMetrics:\n\n");
+	if (!filter || !rblist__empty(&groups)) {
+		if (metricgroups && !raw)
+			printf("\nMetric Groups:\n\n");
+		else if (metrics && !raw)
+			printf("\nMetrics:\n\n");
+	}
 
 	for (node = rb_first_cached(&groups.entries); node; node = next) {
 		struct mep *me = container_of(node, struct mep, nd);