fix: fix server attachment (#4127)

The check excluded an HTTPS server from being properly attached.

Related: #4124

Author: HexaField

lib/index.ts

@@ -156,9 +156,7 @@ export class Server<
     this.adapter(opts.adapter || Adapter);
     this.sockets = this.of("/");
     this.opts = opts;
-    if (srv instanceof http.Server || typeof srv === "number") {
-      this.attach(srv);
-    }
+    if (srv || typeof srv == "number") this.attach(srv as http.Server | number);
   }
 
   /**