Fixed #24844 -- Corrected has_changed implementation for HStoreField.

Backport of 43b2d88a5b from master

django/contrib/postgres/forms/hstore.py

@@ -34,3 +34,13 @@ class HStoreField(forms.CharField):
         for key, val in value.items():
             value[key] = six.text_type(val)
         return value
+
+    def has_changed(self, initial, data):
+        """
+        Return True if data differs from initial.
+        """
+        # For purposes of seeing whether something has changed, None is
+        # the same as an empty dict, if the data or initial value we get
+        # is None, replace it w/ {}.
+        initial_value = self.to_python(initial)
+        return super(forms.HStoreField, self).has_changed(initial_value, data)