[Bug] Unexpected response when bitcoind is syncing

minor issue I noticed but I'm not sure if this is the expected result to get while my `bitcoind` is syncing

In order to reproduce sync bitcoind, get a result from augur then stop bitcoind so it gets unsynced. When you start it back up and connect it to augur while its syncing it gave me this response.

`GET` `http://100.97.107.124:8080/fees`
```
{
"mempool_update_time" : "2025-08-03T16:01:16.626Z",
"estimates" : {
"3" : {
"probabilities" : { }
},
"6" : {
"probabilities" : { }
},
"9" : {
"probabilities" : { }
},
"12" : {
"probabilities" : { }
},
"18" : {
"probabilities" : { }
},
"24" : {
"probabilities" : { }
},
"36" : {
"probabilities" : { }
},
"48" : {
"probabilities" : { }
},
"72" : {
"probabilities" : { }
},
"96" : {
"probabilities" : { }
},
"144" : {
"probabilities" : { }
}
}
}
```

I was expecting the `No fee estimates available yet` error to appear, since having empty objects is equivalent to no estimate

----

Note: I just noticed this on mainnet, but I can see if I can reproduce using regtest and then provide a patch