Exception if the NGSI proxy URL is empty

[aarranz]

NGSI operator raises a TypeError: is not a valid URL. if the ngsiproxy setting is empty. This setting should be required only if the operator is configure to monitor one or more attributes (through the Monitored NGSI Attributes setting). If the operator is not configured to do so, the NGSI proxy URL setting can be empty.

As WireCloud currently does not support to conditionally require a setting. The NGSI proxy URL setting should be configure as optional, and the operator should manage such cases to provide a decent error message.