[BEAM-11578] Fix TypeError in dataflow_metrics has 0 distribution sum (…

…apache#17706)

sdks/python/apache_beam/runners/dataflow/dataflow_metrics.py

@@ -219,7 +219,7 @@ def _get_metric_value(self, metric):
       dist_sum = _get_match(
           metric.distribution.object_value.properties,
           lambda x: x.key == 'sum').value.integer_value
-      if not dist_sum:
+      if dist_sum is None:
         # distribution metric is not meant to use on large values, but in case
         # it is, the value can overflow and become double_value, the correctness
         # of the value may not be guaranteed.