Relayer syncs multiple parachain heads even if no messages are sent

[serban300]

This is causing failures of the 0002-mandatory-headers-synced-while-idle bridge test in the polkadot-sdk repo

After this is fixed, let's not forget to add the test to the CI

Reproduction steps

1. Clone the polkadot-sdk repo locally: https://github.com/paritytech/polkadot-sdk
2. Set all the necessary env variables https://github.com/paritytech/polkadot-sdk/blob/aa68ea58f389c2aa4eefab4bf7bc7b787dd56580/bridges/zombienet/run-new-test.sh#L21-L40
3. Go to the $POLKADOT_SDK_PATH/bridges/zombienet/environments/rococo-westend
4. Run spawn.sh --start-relayer
5. Open the Rococo BH and Westend BH pages in polkadot js apps

Result

On my machine, for example, for Westend BH, the parachain head was updated 3 times even if no messages were sent on the bridge:

[svyatonik]

Fixed in above ^^^ PRs