UPDATES: Fix issue where blank updates do not result in an error

It's not obvious that this is not an allowed action:

If you click "Done" it appears to post but as there's no text nothing happens.

I had one user ask me why their update hadn't posted - honestly I'd never thought of posting this kind of 'update' - but they wanted to quickly notify me of a document they'd just attached to a task, and I can see their logic in doing the above.

Please can this be fixed with an error message? Thank you!