…ssing (#259)

## Explanation

`onAmountInput` currently estimates fees by building a transaction
before the recipient is always available. Closed PR #245 fixed part of
that by accepting `toAddress` in the request and using it for fee
estimation.

This PR ports that fix onto the `fix/incident-1528` branch and adds the
incident-specific safeguard we want on top: when `toAddress` is not
provided, the handler now skips fee validation and returns `valid: true`
after the normal asset-balance check. When `toAddress` is provided, fee
estimation uses the real recipient address.

This prevents fee-estimation failures for token contracts that reject
zero-address transfers while preserving correct fee validation once the
user has entered a recipient.

## References

- Related to
[incident-1528](https://consensys.slack.com/archives/C0AMFS19E0P)
- Adapts the changes from
[#245](#245)

## Checklist

- [x] I've updated the test suite for new or updated code as appropriate
- [x] I've updated documentation (JSDoc, Markdown, etc.) for new or
updated code as appropriate
- [x] I've communicated my changes to consumers by [updating changelogs
for packages I've
changed](https://github.com/MetaMask/core/tree/main/docs/processes/updating-changelogs.md)
- [ ] I've introduced [breaking
changes](https://github.com/MetaMask/core/tree/main/docs/processes/breaking-changes.md)
in this PR and have prepared draft pull requests for clients and
consumer packages to resolve them