Updates:
--------
- Annotate subject attribute as a Union of nullobject, str, None
- Annotate object attribute as a Union of nullobject, str, None


And fix the following errors:
-----------------------------
yams/buildobjs.py:966: error: Incompatible types in assignment (expression has type "str", variable has type "nullobject")  [assignment]
		self.subject = subject
			       ^
yams/buildobjs.py:971: error: Incompatible types in assignment (expression has type "str", variable has type "nullobject")  [assignment]
		self.object = object

yams/buildobjs.py:603: error: Incompatible types in assignment (expression has type "str", variable has type "nullobject")  [assignment]
				relation_definition.subject = name
							      ^
yams/buildobjs.py:606: error: Incompatible types in assignment (expression has type "str", variable has type "nullobject")  [assignment]
				relation_definition.object = name