Skip transaction if no signatures found

[AlaaElattar]

Description

• In case the signatures are gone from the transaction while handling the ready events, we should just ignore the transactions.

Related Issues:

• [Bridge] qanet and testnet bridges keep crashing with no transactions has no signatures error #1038

Checklist:

Please delete options that are not relevant.

• My change requires a change to the documentation and I have updated it accordingly
• My change requires storage migration and I have included and tested it following fork off and try_runtime instructions.
• I have added tests to cover my changes.
• I followed the Release document.
• My commits follow this conventional commits guide.

[sameh-farouk]

We're relying on TFChain's retry mechanism to re-notify us about the related transaction.
Therefore, I suggest renaming the action to withdraw_postponed and refund_postponed.
The corresponding message should also reflect this for example:
"The withdraw has been postponed due to..."
Also, please include a reason metadata to be consistent with other logs, see withdraw_postponed in handleWithdrawReady().
Maybe add refund_postponed to the bridge/docs/observability.md