MFC r321450: 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
Approved by: emaste (mentor, blanket MFC)