bsdgrep: fix zero-length matches without the -o flag

r316477 broke zero-length matches when not using the -o flag, by
skipping over them entirely.

Add a regression test so that it doesn't break again in the future.

Submitted by: Kyle Evans <kevans91 at ksu.edu>
Reviewed by: cem emaste ngie
Differential Revision: https://reviews.freebsd.org/D10333