Get listen.sh on the transfers app to run only when there are requests in the queue

## Summary

The transfers app `listen.sh` script was more or less copied from the `mtcs` app during the hackathon. `mtcs` is expected to run once every few days/weeks, where `transfers` runs on every block. But we clear the entire queue on one listening event, so it calls to the enclave redundantly with an empty Requests vector

## Acceptance Criteria

Get the `listen.sh` script to only run as necessary. There should be no calls to the enclave when the Requests vector is empty.

Find the transfers app's listen.sh and read how it invokes the enclave after a listening event. Verify the current behavior with an empty Requests vector, then make the script skip the enclave call when the queue is empty. Done means no enclave calls occur when Requests has no entries.

Written by the indexing model from the issue text.