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.
# invalid filenames
test-add.t
test-init.t
test-clone.t
test-contrib.t
test-hgweb-raw.t
test-walk.t
# no sockets or fifos
test-hup.t
# no hardlinks
test-hardlinks.t
test-relink.t
# exec bit problems
test-convert-bzr-114.t
test-convert-bzr-directories.t
test-convert-bzr-merges.t
test-convert-bzr-treeroot.t
test-convert-darcs.t
test-merge-tools.t
# debugstate exec bit false positives
test-dirstate.t
test-filebranch.t
test-merge-remove.t