perf jevents: Skip optional metrics in metric group list

For metric groups, skip metrics in the list that are None. This allows
functions to better optionally return metrics.

Signed-off-by: Ian Rogers <[e-mail]>
Tested-by: Thomas Falcon <[e-mail]>
Signed-off-by: Namhyung Kim <[e-mail]>

tools/perf/pmu-events/metric.py

···
   """
 
   def __init__(self, name: str,
-               metric_list: List[Union[Metric, 'MetricGroup']],
+               metric_list: List[Union[Optional[Metric], Optional['MetricGroup']]],
                description: Optional[str] = None):
     self.name = name
-    self.metric_list = metric_list
+    self.metric_list = []
     self.description = description
     for metric in metric_list:
-      metric.AddToMetricGroup(self)
+      if metric:
+        self.metric_list.append(metric)
+        metric.AddToMetricGroup(self)
 
   def AddToMetricGroup(self, group):
     """Callback used when a MetricGroup is added into another."""