Don't update countdown label when there is no label

[joh]

(https://answers.launchpad.net/alarm-clock/+question/228234)

When looking at DBus traffic, we see alarm_clock messages every second:

$ dbus-monitor signal sender=:1.51 -> dest=(null destination) serial=15763 path=/org/ayatana/NotificationItem/alarm_clock; interface=org.kde.StatusNotifierItem; member=XAyatanaNewLabel string "" string ""

etc. etc. etc.

This happen even when there is NO countdown running, and even when there is NO countdown label.

Alarm clock should not throw useless DBus messages.

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Imported from Launchpad using lp2gh.

• date created: 2013-05-09T13:20:56Z
• owner: oluc
• the launchpad url was https://bugs.launchpad.net/bugs/1178259