Fix missing final newline in notmuch search output

Fix missing final newline in notmuch search output

A previous commit to fix json formatting for null results
(0b1ddc5f6652bde99d63d9d553777b3d926694cf) accidentally introduced a
regression that removed trailing newlines for non-json output.  (There
wasn't a good test for this previously, but there is now).  The
problem is due to the fundamental differences in formatting between
the json and non-json outputs.  The only way to fix this was to add a
new formatting field that represents the string to output at the end
of a null result.

All output formatting tests should pass now, (in particular, the 4
recent test failures introduced to show this bug).