create_user: Simplify parameter processing.
create_user: Simplify parameter processing.

We remove a bit of error handling for cases where someone provided
only one of the email and full name parameters, with the benefit of
this being a lot cleaner.

rht/zulip,kou/zulip,zulip/zulip,rht/zulip,rht/zulip,zulip/zulip,andersk/zulip,andersk/zulip,rht/zulip,andersk/zulip,zulip/zulip,zulip/zulip,andersk/zulip,andersk/zulip,andersk/zulip,kou/zulip,zulip/zulip,rht/zulip,rht/zulip,kou/zulip,kou/zulip,kou/zulip,rht/zulip,kou/zulip,andersk/zulip,zulip/zulip,kou/zulip,zulip/zulip