Merge lp://qastaging/~3v1n0/notify-osd/fix-empty-newlines-appending into lp://qastaging/notify-osd

Bubble: don't append new lines when receiving empty bodies

Delay the addition of new lines until there's some real content to append.

To be tested with something like

from gi.repository import Notify
from time import sleep

Notify.init("Test App")

for i in range(10):
    n = Notify.Notification.new("Title", "" if i % 3 else "test {}".format(i))
    n.set_hint_string("append", "true")
    n.show()
    sleep(1)