There are a few places where commit 18a33f72

 replaced valid (nullable)
(optional) annotations with just (optional). That has a different
meaning.

(nullable) (optional) can only be applied to gpointer* parameters, and
means that both the gpointer* and returned gpointer can be NULL. i.e.
The caller can pass in NULL to ignore the return value; and the returned
value can be NULL.

(optional) can be applied to anything* parameters, and means that the
anything* can be NULL. i.e. The caller can pass in NULL to ignore the
return value. The return value cannot be NULL.

Signed-off-by: Philip Withnall <withnall@endlessm.com>