pwpolicy: Allow clearing policy values.

All values for pwpolicy can be cleared with an empty string in IPA CLI,
and this behavior was missing in ansible-freeipa.

As of today, there is an issue in FreeIPA that does not allow clearing
'minlength' policy. The is is tracked by the FreeIPA project through
https://pagure.io/freeipa/issue/9297

Fixes https://bugzilla.redhat.com/show_bug.cgi?id=2150334