Updates:
--------
- Inside Definition class, annotate description attribute as Union of nullobject, str, None


And fix the following errors:
-----------------------------
yams/buildobjs.py:226: error: Incompatible types in assignment (expression has type "str", variable has type "nullobject")  [assignment]
		self.description = " ".join(self.__doc__.split())

yams/buildobjs.py:894: error: Incompatible types in assignment (expression has type "str", variable has type "nullobject")  [assignment]
		cls.description = " ".join(cls.__doc__.split())