Validate all `required` fields are possible to pass

[charjr]

The logic for doing so would look like this:

3.0.X

if any $required is unspecified
   && $additionalProperties is false:
      warn against impossible schema

3.1.X

if any $required is unspecified
   && $additionalProperties is false
   && $patternProperties does not have a match
   || $patternProperties[match] is impossible

patternProperties
additionalProperties