emacs: Fix exception when fetching empty or unconfigured settings

emacs: Fix exception when fetching empty or unconfigured settings

When "notmuch config" is called with the name of an empty or
unconfigured setting, it prints nothing (not even a new line).
Previously, `notmuch-config-get' assumed it would always print a
newline.  As a result, when `notmuch-config-get' was called with the
name of an empty of unconfigured setting, it would attempt to
(substring "" 0 -1) to strip the newline, which would fail with a
(args-out-of-range "" 0 -1) exception.

Fix this by only stripping the newline if there actually is one.