Return null entity instances instead of null for the functions that need to return an entity instance.

Finally fix the player unspectating bug and introduce a new event for when a player spectates none.
Update the global functions that check for player existance to use the new implementation.

source/Base/Algo.cpp

@@ -131,7 +131,7 @@ static const LightObj & Blip_FindBySprID(Int32 sprid)
         }
     }
     // Unable to locate a blip matching the specified identifier
-    return NullLightObj();
+    return Core::Get().GetNullBlip();
 }
 
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