ipauser: Set allow_empty_string for userauthtype and sshpubkey

The parameters userauthtype and sshpubkey allowing to use "" to reset to
the default value.

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. An idempotency issue with sshpubkey has been found with the test
and fixed additionally.