perf stat: Also hide metric-units from JSON when event didn't run

We decided to hide NULL metric-units rather than showing it as "(null)"
when a dependent event for a metric doesn't exist. But on hybrid systems
if the process doesn't hit a PMU you get an empty string metric unit
instead. To make it consistent change all empty strings to NULL.

Note that metric-threshold is already hidden in this case without this
change.

Where a process only runs on cpu_core and never hits cpu_atom:

Before:

  $ perf stat -j -- true
  ...
  {"counter-value" : "<not counted>", "unit" : "", "event" : "cpu_atom/branch-misses/", "event-runtime" : 0, "pcnt-running" : 0.00, "metric-value" : "0.000000", "metric-unit" : ""}
  {"counter-value" : "6326.000000", "unit" : "", "event" : "cpu_core/branch-misses/", "event-runtime" : 293786, "pcnt-running" : 100.00, "metric-value" : "3.553394", "metric-unit" : "of all branches", "metric-threshold" : "good"}
  ...

After:

  ...
  {"counter-value" : "<not counted>", "unit" : "", "event" : "cpu_atom/branch-misses/", "event-runtime" : 0, "pcnt-running" : 0.00}
  {"counter-value" : "5778.000000", "unit" : "", "event" : "cpu_core/branch-misses/", "event-runtime" : 282240, "pcnt-running" : 100.00, "metric-value" : "3.226797", "metric-unit" : "of all branches", "metric-threshold" : "good"}
  ...

Reviewed-by: Ian Rogers <irogers@google.com>
Signed-off-by: James Clark <james.clark@linaro.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Tested-by: Ian Rogers <irogers@google.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Yicong Yang <yangyicong@hisilicon.com>
Link: https://lore.kernel.org/r/20241112160048.951213-3-james.clark@linaro.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

Author: james-c-linaro

tools/perf/util/stat-display.c

@@ -854,7 +854,8 @@ static void printout(struct perf_stat_config *config, struct outstate *os,
 
 	if (run == 0 || ena == 0 || counter->counts->scaled == -1) {
 		if (config->metric_only) {
-			pm(config, os, METRIC_THRESHOLD_UNKNOWN, "", "", 0);
+			pm(config, os, METRIC_THRESHOLD_UNKNOWN, /*format=*/NULL,
+			   /*unit=*/NULL, /*val=*/0);
 			return;
 		}
 
@@ -909,7 +910,7 @@ static void printout(struct perf_stat_config *config, struct outstate *os,
 			perf_stat__print_shadow_stats(config, counter, uval, aggr_idx,
 						      &out, &config->metric_events);
 	} else {
-		pm(config, os, METRIC_THRESHOLD_UNKNOWN, /*format=*/NULL, /*unit=*/"", /*val=*/0);
+		pm(config, os, METRIC_THRESHOLD_UNKNOWN, /*format=*/NULL, /*unit=*/NULL, /*val=*/0);
 	}
 
 	if (!config->metric_only) {