fix notification sent twice bug when printing with octolapse (#13)

add check to make sure that `new_z` height is always more than zero,
this prevents the plugin to send the same notification twice

 Changes to be committed:
	modified:   octoprint_telegram/telegramNotifications.py

octoprint_telegram/telegramNotifications.py

@@ -516,7 +516,7 @@ def is_notification_necessary(self, new_z, old_z):
                 return True
         zdiff = self.main._settings.get_float(["notification_height"])
         if zdiff and zdiff > 0.0:
-            if old_z is None:
+            if old_z is None or new_z < 0:
                 return False
             # check the zdiff
             if abs(new_z - (old_z or 0.0)) >= 1.0: