Fixed: Don't die if Plex watchlist guid node is missing or null

Fixes #7213

src/NzbDrone.Core.Test/Files/plex_watchlist.json

@@ -35,6 +35,11 @@
         "title": "Avengers: Endgame",
         "year": 2019,
         "Guid": []
+      },
+      {
+        "type": "movie",
+        "title": "Avengers",
+        "year": 2005
       }
     ]
   }

src/NzbDrone.Core/ImportLists/Plex/PlexParser.cs

@@ -73,7 +73,7 @@ namespace NzbDrone.Core.ImportLists.Plex
         {
             var scheme = $"{prefix}://";
 
-            return guids.FirstOrDefault((guid) => guid.Id.StartsWith(scheme))?.Id.Replace(scheme, "");
+            return guids?.FirstOrDefault((guid) => guid.Id.StartsWith(scheme))?.Id.Replace(scheme, "");
         }
     }
 }