localedef: unbreak WITHOUT_LOCALES

After 0fa5403d493b ("pkgbase: move locales into their own package") we
need usr.bin/localedef as a bootstrap tool independent on where
WITHOUT_LOCALE was specified as we ALWAYS process C.UTF-8.

At the same time LOCALES= in the local Makefile is empty but
C.UTF-8 with WITHOUT_LOCALES. C.UTF-8 is excluded from FILES, and thus
after the replacement FILES= is set to only .LC_CTYPE which results in
a build failure not knowing how to build that. Tweak the substitution to
replace only non-empty words so that FILES remains harmlessly empty.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D31589