only call tidelineData.editDatum if edited message is a top-level note

This should fix bug reported here: https://trello.com/c/4OyJ9ryd where user gets an error when editing the text of a comment (not a top-level note) in blip.

@jh-bate review at your leisure :)
how about 
_.isEmpty(message.parentMessage) 

this will check for null and empty string??