Fix crash when api_permisions don't have who: section

Default value of that field was wrong, so lets correct this.

This fixes issue #3957

src/ejabberd_access_permissions.erl

@@ -377,6 +377,6 @@ validator() ->
 	fun(Os) ->
 		{proplists:get_value(from, Os, []),
 		 proplists:get_value(who, Os, none),
-		 proplists:get_value(what, Os, [])}
+		 proplists:get_value(what, Os, {none, none})}
 	end),
       [unique]).