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/client/jwt.ts

@@ -104,9 +104,8 @@ export type IsLoggedInResponse =
   | false;
 let __isLoggedIn: IsLoggedInResponse | undefined = undefined;
 export function isLoggedIn(): IsLoggedInResponse {
-  if (__isLoggedIn === undefined) {
-    __isLoggedIn = _isLoggedIn();
-  }
+  __isLoggedIn ??= _isLoggedIn();
+
   return __isLoggedIn;
 }
 function _isLoggedIn(): IsLoggedInResponse {