Federation: sending Null Activity

When the value of a ForeignKey connection is null, it is not encoded on the activity by the sender, relying on the receiver to retrieve the object (#309 )

If it's okay with the spec, I think we should be sending the connection without @id, but with @value: None like so: https://git.startinblox.com/djangoldp-packages/djangoldp/issues/308#note_42639