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/AttackExecution.ts

@@ -97,11 +97,9 @@ export class AttackExecution implements Execution {
       }
     }
 
-    if (this.startTroops === null) {
-      this.startTroops = this.mg
-        .config()
-        .attackAmount(this._owner, this.target);
-    }
+    this.startTroops ??= this.mg
+      .config()
+      .attackAmount(this._owner, this.target);
     if (this.removeTroops) {
       this.startTroops = Math.min(this._owner.troops(), this.startTroops);
       this._owner.removeTroops(this.startTroops);

src/core/execution/DonateGoldExecution.ts

@@ -19,9 +19,7 @@ export class DonateGoldExecution implements Execution {
     }
 
     this.recipient = mg.player(this.recipientID);
-    if (this.gold === null) {
-      this.gold = this.sender.gold() / 3n;
-    }
+    this.gold ??= this.sender.gold() / 3n;
   }
 
   tick(ticks: number): void {

src/core/execution/DonateTroopExecution.ts

@@ -19,9 +19,7 @@ export class DonateTroopsExecution implements Execution {
     }
 
     this.recipient = mg.player(this.recipientID);
-    if (this.troops === null) {
-      this.troops = mg.config().defaultDonationAmount(this.sender);
-    }
+    this.troops ??= mg.config().defaultDonationAmount(this.sender);
     const maxDonation =
       mg.config().maxPopulation(this.recipient) - this.recipient.population();
     this.troops = Math.min(this.troops, maxDonation);

src/core/execution/SAMLauncherExecution.ts

@@ -109,9 +109,7 @@ export class SAMLauncherExecution implements Execution {
       this.player = this.sam.owner();
     }
 
-    if (this.pseudoRandom === undefined) {
-      this.pseudoRandom = new PseudoRandom(this.sam.id());
-    }
+    this.pseudoRandom ??= new PseudoRandom(this.sam.id());
 
     const mirvWarheadTargets = this.mg.nearbyUnits(
       this.sam.tile(),

src/core/execution/SAMMissileExecution.ts

@@ -31,13 +31,11 @@ export class SAMMissileExecution implements Execution {
   }
 
   tick(ticks: number): void {
-    if (this.SAMMissile === undefined) {
-      this.SAMMissile = this._owner.buildUnit(
-        UnitType.SAMMissile,
-        this.spawn,
-        {},
-      );
-    }
+    this.SAMMissile ??= this._owner.buildUnit(
+      UnitType.SAMMissile,
+      this.spawn,
+      {},
+    );
     if (!this.SAMMissile.isActive()) {
       this.active = false;
       return;

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;

src/core/execution/TransportShipExecution.ts

@@ -89,11 +89,9 @@ export class TransportShipExecution implements Execution {
       this.target = mg.player(this.targetID);
     }
 
-    if (this.troops === null) {
-      this.troops = this.mg
-        .config()
-        .boatAttackAmount(this.attacker, this.target);
-    }
+    this.troops ??= this.mg
+      .config()
+      .boatAttackAmount(this.attacker, this.target);
 
     this.troops = Math.min(this.troops, this.attacker.troops());