Issue Description: When using standalone web server (Nginx) with dynmap, the plugin never generates the JSON config file, giving a "Could not retrieve configuration: Not Found" modal when accessing the web client.
And when looking in the standalone dir, there is 0 .json file

The integrated server works perfectly fine, I setup postgres storage, which also runs fine.

• Dynmap Version: Dynmap-3.7-beta-8-spigot
• Server Version: Purpur 1.21.4
• Pastebin of Configuration.txt: https://pastebin.com/vWnGxEpT
• Pastebin of NGINX conf: https://pastebin.com/EpkE9S5P
• Server Host (if applicable): Self Hosted (Crafty Controller)
• Steps to Replicate: Setup Postgres store, configure dynmap to setup standalone server in /var/opt/minecraft/dynmap/, setup nginx server

any solution to this bug?