bsd.man.mk: Handle MANSRC.{TARGET} for MK_MANCOMPRESS=no and empty MANBUILDCAT

Reviewed by: kevans
Fixes: 65f60d715fd9 ("bsd.man.mk: Add a MANSRC.{TARGET} variable")
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D52181