perf stat: Avoid metric-only segv

[ Upstream commit 2543947c ]

Cycles is recognized as part of a hard coded metric in stat-shadow.c,
it may call print_metric_only with a NULL fmt string leading to a
segfault. Handle the NULL fmt explicitly.

Fixes: 088519f3

 ("perf stat: Move the display functions to stat-display.c")
Signed-off-by: Ian Rogers <irogers@google.com>
Reviewed-by: Kan Liang <kan.liang@linux.intel.com>
Cc: K Prateek Nayak <kprateek.nayak@amd.com>
Cc: James Clark <james.clark@arm.com>
Cc: Kaige Ye <ye@kaige.org>
Cc: John Garry <john.g.garry@oracle.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20240209204947.3873294-4-irogers@google.com
Signed-off-by: Sasha Levin <sashal@kernel.org>