Bug: Icinga for Windows writes exception in case no daemons are configured

In case no background daemon is configured, Icinga for Windows will throw an exception "You cannot call a method on va null-valued expression".

It does not interfere with the installation, but ist not nice to see.

Reason:
We call `Get-IcingaBackgroundDaemons` to fetch if the REST-Api is configured, but the operation fails if no daemon is configured and therefor a null object is returned.