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/core/execution/ShellExecution.ts

@@ -23,9 +23,7 @@ export class ShellExecution implements Execution {
   }
 
   tick(ticks: number): void {
-    if (this.shell === undefined) {
-      this.shell = this._owner.buildUnit(UnitType.Shell, this.spawn, {});
-    }
+    this.shell ??= this._owner.buildUnit(UnitType.Shell, this.spawn, {});
     if (!this.shell.isActive()) {
       this.active = false;
       return;