Enable the @typescript-eslint/no-non-null-assertion eslint rule (#1899)

## Description:

Enable the `@typescript-eslint/no-non-null-assertion` eslint rule.

## 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

src/server/GameServer.ts

@@ -662,8 +662,8 @@ export class GameServer {
 
     // Count occurrences of each hash
     for (const client of this.activeClients) {
-      if (client.hashes.has(turnNumber)) {
-        const clientHash = client.hashes.get(turnNumber)!;
+      const clientHash = client.hashes.get(turnNumber);
+      if (clientHash !== undefined) {
         counts.set(clientHash, (counts.get(clientHash) ?? 0) + 1);
       }
     }
@@ -683,8 +683,8 @@ export class GameServer {
     let outOfSyncClients: Client[] = [];
 
     for (const client of this.activeClients) {
-      if (client.hashes.has(turnNumber)) {
-        const clientHash = client.hashes.get(turnNumber)!;
+      const clientHash = client.hashes.get(turnNumber);
+      if (clientHash !== undefined) {
         if (clientHash !== mostCommonHash) {
           outOfSyncClients.push(client);
         }

src/server/MapPlaylist.ts

@@ -103,14 +103,20 @@ export class MapPlaylist {
       const numAttempts = 10000;
       for (let i = 0; i < numAttempts; i++) {
         if (this.shuffleMapsPlaylist()) {
-          log.info(`Generated map playlist in ${i} attempts`);
-          return this.mapsPlaylist.shift()!;
+          log.info(`Generated map playlist in ${i + 1} attempts`);
+          const next = this.mapsPlaylist.shift();
+          if (next !== undefined) return next;
+          log.error("Playlist unexpectedly empty after successful shuffle; using fallback.");
+          return { map: GameMapType.World, mode: GameMode.FFA };
         }
       }
       log.error("Failed to generate a valid map playlist");
     }
-    // Even if it failed, playlist will be partially populated.
-    return this.mapsPlaylist.shift()!;
+    // Even if it failed, playlist may be partially populated.
+    const fallback = this.mapsPlaylist.shift();
+    if (fallback !== undefined) return fallback;
+    log.error("Playlist empty after shuffle failure; using fallback.");
+    return { map: GameMapType.World, mode: GameMode.FFA };
   }
 
   private shuffleMapsPlaylist(): boolean {

src/server/worker/websocket/handler/message/PostJoinHandler.ts

@@ -131,10 +131,11 @@ function handleWinner(
 
   // Add client vote
   const winnerKey = JSON.stringify(clientMsg.winner);
-  if (!gs.winnerVotes.has(winnerKey)) {
-    gs.winnerVotes.set(winnerKey, { ips: new Set(), winner: clientMsg });
+  let potentialWinner = gs.winnerVotes.get(winnerKey);
+  if (potentialWinner === undefined) {
+    potentialWinner = { ips: new Set(), winner: clientMsg };
+    gs.winnerVotes.set(winnerKey, potentialWinner);
   }
-  const potentialWinner = gs.winnerVotes.get(winnerKey)!;
   potentialWinner.ips.add(client.ip);
 
   const activeUniqueIPs = new Set(gs.activeClients.map((c) => c.ip));