Won't support empty label in app settings

[olberger]

As seen in: #2

/opt/yunomonitor/yunomonitor.py -c /opt/yunomonitor/conf/yunomonitor.yml
INFO:root:CHECKING EACH SERVERS...
INFO:root:[localhost] CONFIGURING...
Exception in thread Thread-1:
Traceback (most recent call last):
  File "/usr/lib/python3.9/threading.py", line 954, in _bootstrap_inner
    self.run()
  File "/opt/yunomonitor/yunomonitor.py", line 448, in run
    self.ynh_maps[self.server] = self._load_monitoring_config()
  File "/opt/yunomonitor/yunomonitor.py", line 488, in _load_monitoring_config
    config = generate_monitoring_config()
  File "/opt/yunomonitor/yunomonitor.py", line 778, in generate_monitoring_config
    "label": app_settings['label'],
KeyError: 'label'
INFO:root:FILTERING...
INFO:root:MAILING...

Dunno why an app wouldn't have a label, though :-/