Allow to remove note from inherited attribute

[lmolkova]

I want to be able to remove the parent note when referencing an attribute. E.g. I have

- id: messaging.stuff
  attributes:
    - id: foo
      brief: some attribute
      note: here's a common attribute definition and individual messaging systems should put things like bar and baz here.

then I want to reference the attribute, but I don't need the note - note provided guidance for messaging extensions, I don't need it inside specific system definition.

- id: messaging.kafka    
  attributes:
    - ref: messaging.foo
      brief: kafka-specific precise and accurate brief
      note: # nothing here, brief provided all the details.

this fails with AttributeError: 'NoneType' object has no attribute 'strip'