Fix service user password 'Conversion to String' error

[LordHepipud]

While trying to setup a service user without a password, the Framework will fail during the Start-IcingaAgentInstallWizard command because an empty String is tried to be converted to SecureString.

Expected Behavior

We should set the SecureString properly to $null and ship this with the arguments provided by the installer later on instead of trying to set an empty String object

Current Behavior

We use incorrectly an empty String instead of a $null SecureString object