[Federation] Update section 5. Policy: Metadata parameters with null value must be treated as error

Issue #2069 resolved
Vladimir Dzhuvinov created an issue

Sections 5.1.2 (Policy Operators) and 5.1.5 (Applying Policies) need to be updated to bring the Federation spec in line with the merged https://bitbucket.org/openid/connect/pull-requests/630

(Prohibited use of null as a metadata value)