Don't display a box at all if the note event doesn't include a comment

[saintamh]

This fixes the bug reported in this comment on PR #1401

[tomhughes]

Is there any point sending an email if there is no body? Why are we even allowing comments with no body?

[tomhughes]

Oh this is notes not changeset comments, so it's probably normal when a note is closed?

[saintamh]

Yes exactly, so if you close a note you can choose not to add a comment, and when you reopen a note you don't even have that option and there's never a comment. In both cases I think it's better to just not display a message box at all.

For other notification mails that have a message box (changeset comment, diary comment, direct message), an empty message would be odd on the part of the sender, and we can just carry that oddity across by leaving in the empty message box, I think.