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/SqBase.hpp

@@ -364,6 +364,7 @@ enum EventType
     EVT_PLAYERKEYPRESS,
     EVT_PLAYERKEYRELEASE,
     EVT_PLAYERSPECTATE,
+    EVT_PLAYERUNSPECTATE,
     EVT_PLAYERCRASHREPORT,
     EVT_VEHICLEEXPLODE,
     EVT_VEHICLERESPAWN,