Nullable ObservableArray attributes are not supported

[domenic]

The attribute getter (and setter) steps for observable array attributes don't really work if the attribute is nullable.

We need to allow:

• Spec writers to return null from such attributes
• Spec writers to react to web developers nulling out such attributes (including rejecting such a set)

[bathos]

Currently ObservableArray<T> is expressly forbidden as a nullable type’s inner type, so they’re not yet supported at all, I think.