Enable @typescript eslint/prefer nullish coalescing eslint rule (#1420)

## Description:

Fixes #952 
Enabled @typescript-eslint/prefer-nullish-coalescing rule and worked
through every error, introducing ?? and ??= operators or disabling
errors with inline comments where appropriate, to the best of my
ability.

## Please complete the following:

- [x] I have added screenshots for all UI updates
- [x] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [x] I have added relevant tests to the test directory
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced
- [x] I understand that submitting code with bugs that could have been
caught through manual testing blocks releases and new features for all
contributors

## Please put your Discord username so you can be contacted if a bug or
regression is found:

g_santos_m

src/server/Master.ts

@@ -146,8 +146,9 @@ app.get(
   "/api/env",
   gatekeeper.httpHandler(LimiterType.Get, async (req, res) => {
     const envConfig = {
-      game_env: process.env.GAME_ENV || "prod",
+      game_env: process.env.GAME_ENV,
     };
+    if (!envConfig.game_env) return res.sendStatus(500);
     res.json(envConfig);
   }),
 );