return a None value instead of 0 on exception

Author: echarles

ipykernel/kernelbase.py

@@ -876,7 +876,7 @@ def get_process_metric_value(self, process, name, attribute=None):
         # Avoid littering logs with stack traces
         # complaining about dead processes
         except BaseException:
-            return 0
+            return None
 
     async def usage_request(self, stream, ident, parent):
         reply_content = {}