patch(1): Exit successfully if we're fed a 0-length patch
This change is made in the name of GNU patch compatibility. If GNU patch is
fed a zero-length patch, it will exit successfully with no output. This is
used in at least one port to date (comms/wsjtx), and we break on this usage.
It seems unlikely that anyone relies on patch(1) calling their completely
empty patch garbage and failing, and GNU compatibility is a plus if it helps
with porting, so make the switch.
Reported by: db
MFC after: 2 weeks