[web] Allow thumbnail cache to be optional

See: https://github.com/ente-io/ente/discussions/1449#discussioncomment-9255346

I'm not yet sure what was the case why it was not initialized, hoping to get
some logs for the error when initializing the cache to see how we got to this
state. But meanwhile, ensure that the code works even without the cache.

web/apps/photos/src/services/download/index.ts

@@ -150,7 +150,7 @@ class DownloadManagerImpl {
         this.ensureInitialized();
 
         const key = file.id.toString();
-        const cached = await this.thumbnailCache.get(key);
+        const cached = await this.thumbnailCache?.get(key);
         if (cached) return new Uint8Array(await cached.arrayBuffer());
         if (localOnly) return null;