Have iOS telephony probe store null if no calls were found, to update…

… store time and produce proper health checks.

Sensus.iOS/Probes/Communication/iOSTelephonyProbe.cs

@@ -97,6 +97,10 @@ protected override IEnumerable<Datum> Poll(CancellationToken cancellationToken)
                 _calls.Clear();
             }
 
+            // if we didn't find any calls, return a null to indicate that the poll went through but didn't find anything.
+            if (data.Count == 0)
+                data.Add(null);
+
             return data;
         }
     }

SensusService/Probes/Network/PollingWlanProbe.cs

@@ -39,4 +39,4 @@ public sealed override int DefaultPollingSleepDurationMS
             }
         }
     }
-}
+}

SensusService/Probes/Probe.cs

@@ -188,6 +188,7 @@ public virtual void Start()
 
         public virtual void StoreDatum(Datum datum)
         {
+            // datum is allowed to be null, indicating the the probe attempted to obtain data but it didn't find any (in the case of polling probes).
             if (datum != null)
             {
                 datum.ProtocolId = Protocol.Id;