fix: add fallback method for currency formatter in `useFiatFormatter` hook

[OGPoyraz]

Description

This PR goal is to add a fallback for unknown currency formats to render it ${amount} ${currency} format.

Related issues

Fixes: https://github.com/MetaMask/metamask-extension/issues/24558

Manual testing steps

1. Go to Settings > General
2. Change your currency to Storj
3. Start a send transaction on Ethereum with any value
4. MM doesn't crash and shows amount storj

Screenshots/Recordings

Before

After

Pre-merge author checklist

• [x] I’ve followed MetaMask Coding Standards.
• [x] I've completed the PR template to the best of my ability
• [x] I’ve included tests if applicable
• [x] I’ve documented my code using JSDoc format if applicable
• [X] I’ve applied the right labels on the PR (see labeling guidelines). Not required for external contributors.

Pre-merge reviewer checklist

• [ ] I've manually tested the PR (e.g. pull and build branch, run the app, test code being changed).
• [ ] I confirm that this PR addresses all acceptance criteria described in the ticket it closes and includes the necessary testing evidence such as recordings and or screenshots.