Updates:
--------
- Annotate cardinality attribute as a Union of nullobject, str, None.
- assert cardinality is an instance of str if not an instance of nullobject


To fix the following errors:
----------------------------
yams/buildobjs.py:1071: error: Incompatible types in assignment (expression has type "str", variable has type "nullobject")  [assignment]
		    self.cardinality = "?1"
				       ^
yams/buildobjs.py:1073: error: Incompatible types in assignment (expression has type "str", variable has type "nullobject")  [assignment]
		    self.cardinality = "**"
				       ^
yams/buildobjs.py:1075: error: Argument 1 to "len" has incompatible type "nullobject"; expected "Sized"  [arg-type]
		assert len(cardinality) == 2
			   ^
yams/buildobjs.py:1076: error: Value of type "nullobject" is not indexable  [index]
		assert cardinality[0] in "1?+*"
		       ^
yams/buildobjs.py:1077: error: Value of type "nullobject" is not indexable  [index]
		assert cardinality[1] in "1?+*"