Error creating geometry from value 'null' (String input unrecognized as WKT EWKT, and HEXEWKB.)

[Vayel]

Hi!

Thanks for this package! When used with https://github.com/makinacorpus/django-leaflet and no initial value, GeoJSONFormField raises the follow error:

Error creating geometry from value 'null' (String input unrecognized as WKT EWKT, and HEXEWKB.)

Indeed, the value is None and is dumped by JSONField into "null". The initial value should be jsonfield.forms.InvalidJSONInput(""). It can be set here if undefined: https://github.com/makinacorpus/django-geojson/blob/master/djgeojson/fields.py#L51

[Vayel]

Can be set for model fields as well.

[Vayel]

Actually, it should be set for model fields as well because a field with blank=True will raise the error above.

Can be done with https://docs.djangoproject.com/en/4.0/howto/custom-model-fields/#converting-values-to-python-objects