fallback to NullType when dialect type not found

Author: Rustem

snowdialect.py

@@ -379,9 +379,10 @@ def _get_schema_columns(self, connection, schema, **kw):
                 elif issubclass(col_type,
                                 (sqltypes.String, sqltypes.BINARY)):
                     col_type_kw['length'] = character_maximum_length
+                else:
+                    col_type = sqltypes.NULLTYPE
 
             type_instance = col_type(**col_type_kw)
-
             current_table_pks = schema_primary_keys.get(table_name)
 
             ans[table_name].append({