Improve canonicalisation when properties have no possible values

[Zac-HD]

{'additionalProperties': {'not': {}},
 'maxProperties': 1,
 'properties': {'a': {'not': {}}},
 'type': 'object'}

is considered canonical, but because there are no possible values for the allowed property keys we should instead canonicalise to

{'maxProperties': 0, 'type': 'object'}

This is only valid with additionalProperties: false and no patternProperties, but in those cases we should omit unsatisfiable properties and decrement maxProperties accordingly.