chainHead: Always report discarded items for storage operations (#6760)
This PR ensures that substrate always reports discarded items as zero. This is needed to align with the rpc-v2 spec Closes: https://github.com/paritytech/polkadot-sdk/issues/6683 cc @paritytech/subxt-team --------- Signed-off-by: Alexandru Vasile <[email protected]> Co-authored-by: GitHub Action <[email protected]>