Determine null GA_json

[borgbyte]

Documentation states:

gdk/include/gdk.h

Lines 493 to 499 in adb65ad

	* This must be called before GA_connect/GA_connect_with_proxy.
	* Notifications may arrive on different threads so the caller must ensure
	* that shared data is correctly locked within the handler.
	* The GA_json object passed to the caller must be destroyed by the caller
	* using GA_destroy_json. Failing to do so will result in memory leaks.
	* When the session is disconnected/destroyed, a final call will be made to
	* the handler with a NULL notification.

Currently to know if a GA_json is null I call GA_convert_json_value_to_string which gives the string "null".

Maybe GA_json_is_null or something could be added?