We should add some tests that pass null into parts of the API. There are cases where it doesn't make sense to test, e.g.

     new CSSKeywordValue(null);

since the IDL says CSSKeywordValue expects a string and the bindings code will automatically convert null into "null" anyway. But we should test cases where it is "valid" to pass a null to see if our code correctly handles it. For example, CSSVariableReferenceValue.fallback can be null and we have tests for that.