bsdgrep(1): Don't exit before processing every file

Given an empty pattern (i.e. grep "" A B), bsdgrep(1) would previously exit()
with the appropriate exit code upon encountering an empty file. Likely intended
as an optimization, but this behavior is technically incorrect since an empty
pattern should match every line.

PR: 220924
Reviewed by: emaste, cem (earlier version), ngie
Approved by: emaste (mentor)
Differential Revision: https://reviews.freebsd.org/D11698