PURPUR_DOWNLOAD_URL: unbound variable

[itzg]

Describe the problem

https://discord.com/channels/660567679458869252/660569755320844308/986923544690716702

Introduced by #1563

Container definition

        environment:
            - TZ=Europe/Berlin
            - HOST_OS=Unraid
            - HOST_HOSTNAME=Tower
            - HOST_CONTAINERNAME=minecraft
            - TYPE=PURPUR
            - OPS=CaptaiNiveau
            - ONLINE_MODE=FALSE
            - MODE=survival
            - MEMORY=8G
            - VERSION=1.18.2
            - EULA=true
            - VIEW_DISTANCE=16
            - SPAWN_PROTECTION=0
            - DATAPACKS=/datapacks
            - COPY_CONFIG_DEST=/data

Container logs

minecraft | [init] Running as uid=1000 gid=1000 with /data as 'drwxrwxrwx 1 1000 1000 630 Jun 12 16:46 /data'
minecraft | [init] Resolved version given 1.18.2 into 1.18.2 and major version 1.18
minecraft | [init] Resolving type given PURPUR
minecraft | /start-deployPurpur: line 9: PURPUR_DOWNLOAD_URL: unbound variable`