Requesting a regular transfer transaction as an admin fails

[Nefrin]

Problem:
Sending a POST request to /v1/transactions/transfer with a valid JSON body in the regular transfer form (no sender field) will result in a 500 Internal server error response.

POST /v1/transactions/transfer HTTP/1.1
Host: localhost:8080
Content-Type: application/json
Authorization: Basic QWRtaW46VGVzdFBhc3N3b3JkMQ==
User-Agent: PostmanRuntime/7.15.0
Accept: */*
Cache-Control: no-cache
Postman-Token: 8008289b-c8cf-443f-87cc-2070a2201ac8,6841da77-9abb-4bc1-80c5-5ea2072d1a53
Host: localhost:8080
accept-encoding: gzip, deflate
content-length: 38
Connection: keep-alive
cache-control: no-cache

{
   "receiver": 2,
   "amount": 500
}

Other Symptoms:
The server at this point throws a java.sql.SQLIntegrityConstraintViolationException as a result of a foreign key constraint failing.

 java.sql.SQLIntegrityConstraintViolationException:
 Cannot add or update a child row: a foreign key constraint fails 
(`matohmat`.`transactions`, 
    CONSTRAINT `fk_transaction_sender` 
    FOREIGN KEY (`sender`) 
    REFERENCES `users` (`id`))

Possible cause:
Requesting all admins shows, that all admins have a user_id of 0.

[{
        "id": 3,
        "user_id": 0, /* THIS */
        "balance": 156,
        "last_seen": "2019-07-07T15:18:05",
        "available": true,
        "user_name": "Admin",
        "email": "fachsachft-in@th-nuernberg.de"
    },
    {
        "id": 4,
        "user_id": 0, /* AND THIS */
        "balance": 0,
        "last_seen": "2019-07-07T15:23:54",
        "available": false,
        "user_name": "newadminyadp6lhg",
        "email": "newadminyadp6lhg@th-nuernberg.de"
    }]

This in itself is an invalid value as IDs start at 1. Using this 0 value as sender ID will make the foreign key constraint fail for the same reason.
Root cause of this might be me failing to implement admin-user creation correctly, missing the read-back of the created users id into the admin user_id