macosx: fix handling of null sub category items in prefs tree construction

subCategoryItem would be null if:
 a) a plugin author neglected to place a set_category() call before one
    or more options.
 b) the first or only subcat targetted by a plugin is a general one.

there are actual examples of A with a handful of in-tree plugins (to be
fixed in a subsequent commit).

there are multiple in-tree modules which target general subcats,
including various logger and keystore plugins.

Signed-off-by: Felix Paul Kühne <felix@feepk.net>