Merge pull request #1731 from van4oza/patch-1

Insert null values fix

mongoengine/base/document.py

@@ -147,7 +147,7 @@ class BaseDocument(object):
 
             if not hasattr(self, name) and not name.startswith('_'):
                 DynamicField = _import_class('DynamicField')
-                field = DynamicField(db_field=name)
+                field = DynamicField(db_field=name, null=True)
                 field.name = name
                 self._dynamic_fields[name] = field
                 self._fields_ordered += (name,)
@@ -337,7 +337,7 @@ class BaseDocument(object):
                 value = field.generate()
                 self._data[field_name] = value
 
-            if value is not None:
+            if (value is not None) or (field.null):
                 if use_db_field:
                     data[field.db_field] = value
                 else: