fix(client): send an empty message in PING - #3464

Closes #3463

ping('') went out as a bare PING. The guard was if (message), and '' is falsy, so the message was never pushed onto the wire and the server answered PONG instead of "".

await client.ping('');   // before: "PONG"   after: ""
await client.ping();     // still "PONG" - the omitted form is unchanged

Change

   parseCommand(parser: CommandParser, message?: RedisArgument) {
     parser.push('PING');
-    if (message) {
+    if (message !== undefined) {
       parser.push(message);
     }
   },

An empty message is not an omitted message - '' is a valid RedisArgument and Redis accepts PING '', which is why the raw sendCommand(['PING', '']) path always worked. The sibling ECHO never had this guard, and XREAD/SINTERCARD already compare against undefined for the same reason.

Tests

packages/client/lib/commands/PING.spec.ts:

• parseArgs(PING, '') now asserts ['PING', ''].
• The ping integration test additionally asserts that await client.ping('') returns ''. It runs on both the server and cluster topologies through testUtils.testAll.

Verification

• npm run build
• npx mocha -r tsx './lib/RESP/**/*.spec.ts' - 133 passing
• npm test -w @redis/client - 3079 passing, 4 pending, 2 failing; both failures are Socket socketTimeout timeouts in socket.spec.ts, unrelated to this change (details below)
• npm run lint - clean

1) Socket
     socketTimeout
       should timeout with positive socketTimeout values:
   Error: Timeout of 2000ms exceeded.
2) Socket
     socketTimeout
       should not timeout with undefined socketTimeout:
   Error: Timeout of 2000ms exceeded.

Checklist

• Does npm test pass with this change (including linting)?
• Is the new or changed code fully tested?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?

No documentation change: ping(message?: RedisArgument) is unchanged, and the fix makes the implementation match what the signature already promised.