perf metrics: Avoid segv with --topdown for metrics without a group

perf metrics: Avoid segv with --topdown for metrics without a group

Some metrics may not have a metric_group which can result in segvs
with "perf stat --topdown". Add a condition for the no metric_group
case.

Fixes: 1647cd5b8802698f ("perf stat: Implement --topdown using json metrics")
Reported-by: Kan Liang <kan.liang@linux.intel.com>
Signed-off-by: Ian Rogers <irogers@google.com>
Tested-by: Kan Liang <kan.liang@linux.intel.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kajol Jain <kjain@linux.ibm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20230515224530.671331-1-irogers@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>