fix: check the format of the event name

A packet like '2[{"toString":"foo"}]' was decoded as:

{
  type: EVENT,
  data: [ { "toString": "foo" } ]
}

Which would then throw an error when passed to the EventEmitter class:

> TypeError: Cannot convert object to primitive value
>    at Socket.emit (node:events:507:25)
>    at .../node_modules/socket.io/lib/socket.js:531:14

History of the isPayloadValid() method:

- added in [78f9fc2](78f9fc2) (v4.0.1, socket.io@3.0.0)
- updated in [1c220dd](1c220dd) (v4.0.4, socket.io@3.1.0)

lib/index.ts

@@ -275,7 +275,10 @@ export class Decoder extends Emitter<{}, {}, DecoderReservedEvents> {
         return typeof payload === "string" || typeof payload === "object";
       case PacketType.EVENT:
       case PacketType.BINARY_EVENT:
-        return Array.isArray(payload) && payload.length > 0;
+        return (
+          Array.isArray(payload) &&
+          (typeof payload[0] === "string" || typeof payload[0] === "number")
+        );
       case PacketType.ACK:
       case PacketType.BINARY_ACK:
         return Array.isArray(payload);

test/parser.js

@@ -118,6 +118,9 @@ describe("socket.io-parser", () => {
     isInvalidPayload("1/admin,{}");
     isInvalidPayload('2/admin,"invalid');
     isInvalidPayload("2/admin,{}");
+    isInvalidPayload('2[{"toString":"foo"}]');
+    isInvalidPayload('2[true,"foo"]');
+    isInvalidPayload('2[null,"bar"]');
 
     expect(() => new Decoder().add("999")).to.throwException(
       /^unknown packet type 9$/