Fixes #12561
When a ModelForm is constructed without passing an `initial` instance, the result of `StreamField.get_default()` will become the default value of the form field without the intermediate step of being set on a model instance and read back (which would have the side effect of calling `normalize()`). Form rendering only works with normalized values (e.g. StreamValue rather than list-of-tuples for a StreamBlock), so the return value from `get_default()` needs to be normalized.