emacs: Fix mail composition under Emacs 23

Commit 570c0aeb40bd0c3af8174624a55e968f62c44f09 reworked
notmuch-mua-mail function in a way that worked only under Emacs 24.
The reason was that message-setup-1 took one argument less in Emacs
23.

We fix this by only supplying the return-action argument when it is
actually set by the caller.