correct form completion workflow for forms in root

http://manage.dimagi.com/default.asp?125219
I agree with [Cory](https://github.com/dimagi/commcare-hq/pull/3910#discussion_r15283482), this seems better. However, could this possibly replace a non-empty list with an empty one? That is, if `datums[module_id][form_id]` had already been populated in a previous iteration. It seems possible if multiple suite entries could reference the same module and form.