fix: on exception, return a 0, so that the "sum" still computes (#1204)

ipykernel/kernelbase.py

@@ -1049,7 +1049,7 @@ def get_process_metric_value(self, process, name, attribute=None):
         # Avoid littering logs with stack traces
         # complaining about dead processes
         except BaseException:
-            return None
+            return 0
 
     async def usage_request(self, stream, ident, parent):
         """Handle a usage request."""