Avoid unnecessary state reset of `allowed_requests` when no block requests are sent (#5774)
This PR is cherry-picked from https://github.com/paritytech/polkadot-sdk/pull/5663 so that I can maintain a smaller polkadot-sdk diff downstream sooner than later. cc @lexnv @dmitry-markin --------- Co-authored-by: Alexandru Vasile <[email protected]> Co-authored-by: Dmitry Markin <[email protected]>