Fixed #20484 -- Added model validation for GenericIPAddressField

GenericIPAddressField must not allow blank for NOT NULL fields

Thanks Erik Romijn.

docs/releases/1.6.txt

@@ -601,6 +601,13 @@ Miscellaneous
   ``django.contrib.admindocs.middleware`` because it is an implementation
   detail of admindocs, proven not to be reusable in general.
 
+* :class:`~django.db.models.GenericIPAddressField` will now only allow
+  ``blank`` values if ``null`` values are also allowed. Creating a
+  ``GenericIPAddressField`` where ``blank`` is allowed but ``null`` is not
+  will trigger a model validation error because ``blank`` values are always
+  stored as ``null``. Previously, storing a ``blank`` value in a field which
+  did not allow ``null`` would cause a database exception at runtime.
+
 Features deprecated in 1.6
 ==========================