Bug 1461221 - Account for psutil.disk_io_counters() returning None; r=mshal

Apparently it can do this.

MozReview-Commit-ID: 6gMTGtcRAw6

--HG--
extra : rebase_source : 1e89583b3c15c85d1c86969f101358864c0e2e34

testing/mozbase/mozsystemmonitor/mozsystemmonitor/resourcemonitor.py

@@ -65,6 +65,9 @@ except Exception:
 def get_disk_io_counters():
     try:
         io_counters = psutil.disk_io_counters()
+
+        if io_counters is None:
+            return PsutilStub().disk_io_counters()
     except RuntimeError:
         io_counters = PsutilStub().disk_io_counters()