new world record_date field omit if empty

[tobiasehlert]

A new world that has no record yet should not display a record date string.. it should rather be omit since it's empty.

Example on /v4/world/Oceanis

{
  "world": {
    "name": "Oceanis",
    "status": "online",
    "players_online": 0,
    "record_players": 0,
    "record_date": "",
    "creation_date": "2024-04",
...