## Description 

Currently if the exchange rate for an epoch is not found, we would
assign it the default exchange rate while we should've used the exchange
rate from the previous existing epoch instead. This PR fixes that.

## Test plan 

Added some unit tests.

---

## Release notes

Check each box that your changes affect. If none of the boxes relate to
your changes, release notes aren't required.

For each box you select, include information after the relevant heading
that describes the impact of your changes that a user might notice and
any actions they must take to implement updates.

- [ ] Protocol: 
- [ ] Nodes (Validators and Full nodes): 
- [ ] Indexer: 
- [x] JSON-RPC: fixed reward estimation in `getStakes` API so that we
don't overestimate stake rewards of stakes activated during safe mode
epochs.
- [ ] GraphQL: 
- [ ] CLI: 
- [ ] Rust SDK:
- [ ] REST API: