Fix Swagger generation for array of embedded objects with empty array as default value

[pidhorodetskyi]

If response dto has property $entities with type Entity[] and with default value as empty array
swagger file will be generated with empty array and not with Entity[] schema

{
 "entities": []
}

This part of code will replace empty array with Generator::UNDEFINED
and swagger file will generate proper Entity[] schema

{
    "entities": [
        {
            "propertyId": 0,
            "branchId": 0,
            ....
        }
    ]
}