emacs: add invisible dot instead of space at the end of notmuch-hello search box

This makes `show-trailing-whitespace' happy, i.e. it does not mark the
whole search box line as trailing spaces.

Since the dot is invisible, this change makes no visible difference
for `notmuch-hello'.

Edited-by: Pieter Praet <pieter@praet.org> to fix the tests.