reply: Remove extraneous space from generated References

Previously, the References header code seemed to assume
notmuch_message_get_header would return NULL if the header was not
present, but it actually returns "".  As a result of this, it was
inserting an unnecessary space when concatenating an empty or missing
original references header with the new reference.

This shows up in only two tests because the text reply format later
passes the whole reply template through g_mime_filter_headers, which
has the side effect of stripping out this extra space.