introspection: add missing (nullable) annotations to return values

introspection: add missing (nullable) annotations to return values

Support for (nullable) was added to G-I at the same time as nullable
return values.  Previous versions of G-I will not mark return values as
nullable, even when an (allow-none) annotation is present, so it is
not necessary to add (allow-none) annotations for compatibility with
older versions of G-I.

https://bugzilla.gnome.org/show_bug.cgi?id=730957