Allow first=None for add_implicit_resolver.
Allow first=None for add_implicit_resolver.

jamesbroadhead/pyyaml,yaml/pyyaml,jamesbroadhead/pyyaml,yaml/pyyaml,anishathalye/pyyaml,fuchsia-mirror/third_party-pyyaml,yaml/pyyaml,anishathalye/pyyaml,fuchsia-mirror/third_party-pyyaml