Hide complex types after applying role leaves it empty

This only applies if field or model has `serialize_when_none` set to False.
Merge branch 'hide-multitypes-when-empty' into jokull

* hide-multitypes-when-empty:
  Hide complex types after applying role leaves it empty
Raise ValidationError when using partial submodel

Until a good usecase for partial submodels appears, this is the
documented behaviour, which was previously not followed when using
a Model instance directly. This patch adds the testcase and a fix.
Merge branch 'validation-error-on-partial-submodel' into jokull

* validation-error-on-partial-submodel:
  Raise ValidationError when using partial submodel