Fix header file generation to not be confused by empty (but non-NULL) option string.

You shouldn't use "" as an option string because a NULL allows toy_init()
to avoid calling the option parsing logic entirely, which lets it drop out
when nothing's using it (ala scripts/single.sh builds). That said, init.c
is currently using a "" option string and a USE_ macro taking advantage
of string concatenation may require a "" option config, so...