Just some observation with respect to the mysterious "empty" tag. In
notmuch-show, if I press +<RET>, you can see the tag line changing, so
it has been adding something (a newline?). You can remove it again with
-<RET>.

IMHO we should protect against adding this from the emacs UI. I
sometimes press accidentally +, and want to get out of the dialog by
pressing return (rather than c-g).

Sebastian