The macros differ in their handling of NULL values — some macros ignore
them and pass through (e.g. G_TYPE_CHECK_INSTANCE_CAST) while others
will explicitly emit a warning if passed NULL (e.g.
G_TYPE_CHECK_INSTANCE).
Document their behaviour, so people don’t end up putting unnecessary
NULL checks in their code when doing checked type casts.
https://bugzilla.gnome.org/show_bug.cgi?id=735731