[Bug]: Ensure we check for nonempty pre-sign requests

[djordon]

Bug - Ensure we check for nonempty pre-sign requests

1. Description

The signers do not check whether the pre-sign request contains a non-empty list of deposit or withdrawal requests. Although the coordinator does not construct bitcoin transactions when there aren't any requests to handle, the other signers do not enforce this and they should.

1.1 Context & Purpose

There is no need to allow pre-sign requests to be devoid of user requests, since the signers only act when there is user input. This is because acting costs money in transaction fees, and there is no need to spend any money when you do not need to.

2. Technical Details:

The fix is to check that the request_package field in the pre-sign request is not empty and that each TxRequestIds in that field is non-empty.

2.1 Acceptance Criteria:

• We do not allow for the creation of a bitcoin transaction that does not service user requests.

3. Related Issues and Pull Requests (optional):