/v2/transactions/params "fee" does not go down until a transaction is submitted

### Subject of the issue

If the fee per byte kicks in, /v2/transactions/params "fee" will continuously report the last reported fee-per-byte until a new transaction is submitted

### Your environment

Local sandnet using [docker](https://github.com/joe-p/docker-algorand/tree/ad5cad70557a353fcc2afe6d3b52e566d83c4f61) and with version `3.9.4.dev [rel/stable] (commit #921e8f6f)`

This has also been reproduced on a tesnet node

### Steps to reproduce

1. Spam transactions until fee-per-byte kicks in
2. Wait for fee-per-byte to go back to 0

### Expected behaviour
The fee-per-byte goes to 0 once the transaction pool clears out

### Actual behaviour
The fee-per-byte stays at whatever the last fee-per-byte was until a new txn is submitted to the node

### Example
The following code produces an infinite loop
```py
while(app_client.client.suggested_params().fee > 1):
print("Waiting for fee to drop...")
time.sleep(1)
```

Looking directly at the HTTP endpoints:

```
$ curl localhost:4001/v2/transactions/params -H "X-Algo-API-Token: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
{"consensus-version":"https://github.com/algorandfoundation/specs/tree/433d8e9a7274b6fca703d91213e05c7e6a589e69","fee":2,"genesis-hash":"z2kpIJUTMU2sMsVaIpW+Rt9Qh5/b3n2GJq49xApJs1Q=","genesis-id":"sandnet-v1","last-round":660,"min-fee":1000}

$ curl localhost:4001/v2/transactions/pending -H "X-Algo-API-Token: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
{
"top-transactions": [],
"total-transactions": 0
}
```