[Toybox] [PATCH] grep: `-f empty-file` matches nothing, not everything.

[enh]

This came up trying to build kernels. Worked around for now by
https://android-review.googlesource.com/c/kernel/build/+/1558655. POSIX
says nothing about this, but the GNU grep man page explicitly mentions
this behavior.

Bug: https://issuetracker.google.com/178191979
---
 tests/grep.test   |  8 ++++++++
 toys/posix/grep.c | 25 +++++++++++++++----------
 2 files changed, 23 insertions(+), 10 deletions(-)
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