patchbomb: treat empty address list as no addresses
Previously it wasn't possible to use configuration to avoid
being prompted for e.g. a CC list when using patchbomb to send
emails.
We now make it possible to supply an empty value.
#require test-repo
$ cd "$TESTDIR"/..
New errors are not allowed. Warnings are strongly discouraged.
$ hg files "set:(**.py or **.txt) - tests/**" | sed 's|\\|/|g' |
> xargs python contrib/check-config.py