Fix overzealous validation of name_id_format

The configuration document states that name_id_format can be "any valid saml name id format or shortcuts: persistent or transient", but this config validation only allows "persistent" and "transient". This commit relaxes the validation (actually it just sets the default and disallows empty).

An alternative fix would be to leave it as an enum and add the supported raw identifiers (e.g. `->values(array('persistent', 'transient', 'urn:oasis:names:tc:SAML:2.0:nameid-format:persistent', 'urn:oasis:names:tc:SAML:2.0:nameid-format:transient', 'urn:oasis:names:tc:SAML:1.1:nameid-format:emailAddress', 'urn:mace:shibboleth:1.0:nameIdentifier'))`.