fix: 0 token balance in send flow (#28136)

## **Description**

Fixes an issue where token balance showed as 0 during send flow. This
occurred when clicking the token in the token list, then clicking the
send button from the token details page.

When going send first and then picking a token, picking `decimals` was a
number:


![image](https://github.com/user-attachments/assets/95d32a68-1076-4050-b129-187672694794)

But when going token first and then clicking send , `decimals` was a
string and skipped calculating the balance.


![image](https://github.com/user-attachments/assets/99a8fcd4-bd29-4969-b98d-6639d50553ff)


`calcTokenAmount` seems to work with either string or number, so
changing logic from
#27083 which
introduced the number check


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## **Related issues**

Fixes: #28112

## **Manual testing steps**

1. Click on an erc20 token on the token list
2. Click the send button on the token details page
3. Choose a destination account
4. The balance under the asset picker should be accurate

## **Screenshots/Recordings**

<!-- If applicable, add screenshots and/or recordings to visualize the
before and after of your change. -->

### **Before**

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### **After**

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## **Pre-merge author checklist**

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Docs](https://github.com/MetaMask/contributor-docs) and [MetaMask
Extension Coding
Standards](https://github.com/MetaMask/metamask-extension/blob/develop/.github/guidelines/CODING_GUIDELINES.md).
- [ ] I've completed the PR template to the best of my ability
- [ ] I’ve included tests if applicable
- [ ] I’ve documented my code using [JSDoc](https://jsdoc.app/) format
if applicable
- [ ] I’ve applied the right labels on the PR (see [labeling
guidelines](https://github.com/MetaMask/metamask-extension/blob/develop/.github/guidelines/LABELING_GUIDELINES.md)).
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.

ui/ducks/send/send.js

@@ -2670,7 +2670,7 @@ export function updateSendAsset(
 
       if (details.standard === TokenStandard.ERC20) {
         asset.balance =
-          details.balance && typeof details.decimals === 'number'
+          details.balance && details.decimals !== undefined
             ? addHexPrefix(
                 calcTokenAmount(details.balance, details.decimals).toString(16),
               )