commit: make --only --allow-empty work without paths

commit: make --only --allow-empty work without paths

--only is implied when paths are present, and required
them unless --amend. But with --allow-empty it should
be allowed as well - it is the only way to create an
empty commit in the presence of staged changes.

Signed-off-by: Andreas Krey <redacted>
Reviewed-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>