[Feature] JSON generation should allow false subschemas to blacklist keys

[hudson-ai]

E.g. the following schema should allow the generation of any object that does not contain the key "name":

{
 "type": "object",
 "properties": {
   "name": false
 },
 "required": []
}

[hudson-ai]

Note that

{
 "type": "object",
 "properties": {
   "name": false
 },
 "required": ["name"]
 }

should still raise an exception because there are no valid instances of this schema