Add support for object nullability expressed with "oneOf"

[tomasz-dudziak]

There are three different ways in JSON schema to say that a field is a nullable object. One of them is for example:

"myField1" : {
                "oneOf": [
                  {
                    "type": "null"
                  },
                  {
                    "type": "object",
                    "properties": {
                      "prop" : {
                        "type" : "string"
                      }
                    }
                  }
                ]
              }

This is currently not supported.