fix: do not poll log entries without lastReqId

[Antreesy]

• Fix Duplicate log messages in browser after first js refresh #1271
• According to LogController.php#poll(), lastReqId should be a string. Frontend passes an empty string as a fallback, which supposedly iterate all the logs to the bottom (because there is no match)

Alternative (and workaround) would be to add check to the backend, like:

	public function poll(string $lastReqId): JSONResponse {
		if ($lastReqId === '') {
			return new JSONResponse([]);
		}

But we simply should not make such request from the client

[Antreesy]

/compile

[susnux]

makes sense

[Antreesy]

/backport to stable32