bsdgrep(1): Fix zero-length matches when not using the -o flag

[kevans]

Summary

r316477 broke zero-length matches when not using the -o flag, by skipping over them entirely. When trying to match non-empty lines (i.e. grep -v -e "^$" [file]), this causes problems.

This is likely to break many things in itself, add a regression test for it while I'm here so I don't break it in the future.

Test Plan

Run kyua tests to ensure no further regressions, especially with -o flag behavior.

[emaste]

FYI if you use the 'rS' prefix on a SVN rev Phabricator will link it, e.g. rS316477