Changes in pyxform behavior break submission editing

[jnm]

Consider:

• person makes and deploys a form
• KoBoCAT uses pyxform to convert the form to XML XForm
• KoBoCAT stores the resulting XML in its Postgres database
• person collects a submission with that form
• we upgrade pyxform to a new version that forbids something¹ done in that form
• person now tries to edit a submission they collected:
  • if they use kobocat, it works! the change in pyxform behavior doesn't matter because the XML XForm is stored in KoBoCAT's database
  • if they use KPI (Open edit/preview Enketo Express links with KPI as OpenRosa server #3689), it breaks! KPI does not store the XML XForm permanently. when the edit request is made, KPI invokes pyxform in an attempt to generate the XML XForm, and it fails.

¹ allow_choice_duplicates: XLSForm/pyxform#23; we didn't bring in this change until May 2021

Result:
Person inexplicably (from their perspective) cannot edit their submissions. They see a cryptic error: Start tag expected, '<' not found.

[jnm]

@joshuaberetta found that when this happens, KPI serves an empty document to Enketo. This also happens when attempting to download a form as XML from KPI.