Fix stream status for absent streams (#39)

Signed-off-by: Dave Thaler <[email protected]>

OrcanodeMonitor/Models/Orcanode.cs

@@ -219,7 +219,7 @@ public string OrcaHelloName
 
         private static OrcanodeOnlineStatus GetOrcasoundOnlineStatus(string slug, DateTime? manifestUpdatedUtc, DateTime? lastCheckedUtc)
         {
-            if (slug == null)
+            if (slug.IsNullOrEmpty())
             {
                 return OrcanodeOnlineStatus.Absent;
             }