Bug fix: For XML serialization, UserPassword in passwordless scenario

Using the --no-ssh-password option still fails because of a removed line after feedback - turns out the removed line is key!

When in passwordless mode, the UserPassword value in the prov config needs to be initialized to an empty string.

This allows the XML serialization code to behave OK. It does not accept undefined as a value (the xml serl. module we use).

This code was unfortunately removed last minute last time. Works now. Sorry!