Address test failures regarding form POSTs
Address test failures regarding form POSTs

Django 2.2's test client does not support serializing None values as
multipart/form-data, because there is no valid way to represent None.
Previous versions of Django coerced None into the string "None" which
was prone to hiding errors. Now it throws instead.

See https://code.djangoproject.com/ticket/30024

Our tests pass once we filter out keys with `None` values

Fixes #6566

mozilla/kuma,Elchi3/kuma,escattone/kuma,mozilla/kuma,a2sheppy/kuma,mozilla/kuma,mozilla/kuma,a2sheppy/kuma,a2sheppy/kuma,escattone/kuma,Elchi3/kuma,Elchi3/kuma,Elchi3/kuma,a2sheppy/kuma,Elchi3/kuma,mozilla/kuma,escattone/kuma,a2sheppy/kuma