The GSourceFuncs documentation states 'The dispatch function receives a callback function and user data. The callback function may be NULL if the source was never connected to a callback using g_source_set_callback().' This merge request reflects that in the glib-2.0 binding.
It breaks the API binding from the Vala side so probably one for 0.42 so people can wrap it in
#if VALA_0_42. Although I'm not sure if anyone is using custom GSources in Vala.