perf stat: Drop metric-unit if unit is NULL

Avoid cases like:
```
$ perf stat -a -M topdownl1 -j -I 1000
...
{"interval" : 11.127757275, "counter-value" : "85715898.000000", "unit" : "", "event" : "IDQ.MITE_UOPS", "event-runtime" : 988376123, "pcnt-running" : 100.00, "metric-value" : "0.000000", "metric-unit" : "(null)"}
...
```

If there is no unit then drop the metric-value too as:
Suggested-by: Tim Chen <tim.c.chen@linux.intel.com>

Signed-off-by: Ian Rogers <irogers@google.com>
Cc: Yicong Yang <yangyicong@hisilicon.com>
Cc: Weilin Wang <weilin.wang@intel.com>
Cc: Will Deacon <will@kernel.org>
Cc: James Clark <james.clark@linaro.org>
Cc: Mike Leach <mike.leach@linaro.org>
Cc: Leo Yan <leo.yan@linux.dev>
Cc: Sumanth Korikkar <sumanthk@linux.ibm.com>
Cc: Thomas Richter <tmricht@linux.ibm.com>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Cc: John Garry <john.g.garry@oracle.com>
Link: https://lore.kernel.org/r/20241017175356.783793-5-irogers@google.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>

1 files changed, 2 insertions, 2 deletions

diff --git a/tools/perf/util/stat-display.c b/tools/perf/util/stat-display.c
index a137b377efa4..c6d33010ed22 100644
--- a/tools/perf/util/stat-display.c
+++ b/tools/perf/util/stat-display.c
@@ -471,8 +471,8 @@ static void print_metric_json(struct perf_stat_config *config __maybe_unused,
 	struct outstate *os = ctx;
 	FILE *out = os->fh;
 
-	fprintf(out, "\"metric-value\" : \"%f\", ", val);
-	fprintf(out, "\"metric-unit\" : \"%s\"", unit);
+	if (unit)
+		fprintf(out, "\"metric-value\" : \"%f\", \"metric-unit\" : \"%s\"", val, unit);
 	if (!config->metric_only)
 		fprintf(out, "}");
 }