If we get a notification with no body, treat the subject line as the body

Summary: BUG: 372112

Test Plan:
notify-send "This long title that goes on and on" - wrapped with no
title

notify-send "subject1" "msg1"
notify-send "subject1" "msg2" - appeared as one message with two lines

notify-send "subject2" "msg1"
notify-send "subject3" "msg2" - appeared as two notifications

Reviewers: Plasma

Subscribers: mart, broulik, apol, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D3425