ipaconfig: Set allow_empty_string for user_auth_type, pac_type, configstring

The parameters user_auth_type, pac_type and configstring are allowing to
use "" to reset to the default value or for configstring to set an empty
list.

The new check in params_get is not allowing to use empty strings in lists,
therefore allow_empty_string=True had to be added to the call.

A test has been added to verify that the empty strings are supported and
working.

Additionally empty pac_type, user_auth_type and domain_resolution_order
have been added to exit_args as if they have not been set.