It turned out that the new restrictions on Instance creation (i.e., both a username and XForm uuid required) were too strict, since older XLSforms and their corresponding xml files do not have any such uuid defined.

The only thing those have is an id_string, so this commit changes the logic to allow for retrieving an XForm by either:

(a) The given XForm uuid (preferred case); or
(b) The id_string/username combination