Fixed #16110 -- Fixed GeometryField odd behaviour regarding null values

Thanks slinkp for the report and the initial patch.

django/contrib/gis/forms/fields.py

@@ -1,5 +1,7 @@
 from __future__ import unicode_literals
 
+import warnings
+
 from django import forms
 from django.utils import six
 from django.utils.translation import ugettext_lazy as _
@@ -18,7 +20,7 @@ class GeometryField(forms.Field):
     widget = forms.Textarea
 
     default_error_messages = {
-        'no_geom' : _('No geometry value provided.'),
+        'required' : _('No geometry value provided.'),
         'invalid_geom' : _('Invalid geometry value.'),
         'invalid_geom_type' : _('Invalid geometry type.'),
         'transform_error' : _('An error occurred when transforming the geometry '
@@ -30,13 +32,18 @@ class GeometryField(forms.Field):
         # defaults (e.g., allow None).
         self.srid = kwargs.pop('srid', None)
         self.geom_type = kwargs.pop('geom_type', 'GEOMETRY')
-        self.null = kwargs.pop('null', True)
+        if 'null' in kwargs:
+            kwargs.pop('null', True)
+            warnings.warn("Passing 'null' keyword argument to GeometryField is deprecated.",
+                DeprecationWarning, stacklevel=2)
         super(GeometryField, self).__init__(**kwargs)
 
     def to_python(self, value):
         """
         Transforms the value to a Geometry object.
         """
+        if value in self.empty_values:
+            return None
         try:
             return GEOSGeometry(value)
         except (GEOSException, ValueError, TypeError):
@@ -48,15 +55,9 @@ class GeometryField(forms.Field):
         object (which is returned).  A ValidationError is raised if
         the value cannot be instantiated as a Geometry.
         """
-        if not value:
-            if self.null and not self.required:
-                # The geometry column allows NULL and is not required.
-                return None
-            else:
-                raise forms.ValidationError(self.error_messages['no_geom'])
-
-        # Transform the value to a python object first
-        geom = self.to_python(value)
+        geom = super(GeometryField, self).clean(value)
+        if geom is None:
+            return geom
 
         # Ensuring that the geometry is of the correct type (indicated
         # using the OGC string label).