fix: standardize fallback value for zero hex in balance checks (#38649)

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## **Description**

This PR fixes a [issue raised by
cursor](#38306 (comment))
to 0x0 is a numeric literal (the number zero) rather than the hex string
'0x0'

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## **Related issues**

Fixes: MetaMask/MetaMask-planning#6408

## **Manual testing steps**

1. Send flow with insufficient  balance
2. should display the insufficient alert 

## **Screenshots/Recordings**


[alert.webm](https://github.com/user-attachments/assets/5cf66e88-8256-4995-9b2d-4cb30f2f5198)


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<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Standardizes zero-hex fallback via a shared `ZERO_HEX_FALLBACK` in
`useHasInsufficientBalance` for tx value, batch values, and balance
lookups.
> 
> - **Hooks**
>   - `ui/pages/confirmations/hooks/useHasInsufficientBalance.ts`:
>     - Introduces `ZERO_HEX_FALLBACK = '0x0'`.
> - Replaces inline zero-hex defaults with `ZERO_HEX_FALLBACK` for
`txParams.value`, nested transaction values, and balance fallback.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
3d24dbb. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: vinistevam

ui/pages/confirmations/hooks/useHasInsufficientBalance.ts

@@ -11,6 +11,8 @@ import {
 import { useConfirmContext } from '../context/confirm';
 import { isBalanceSufficient } from '../send-legacy/send.utils';
 
+const ZERO_HEX_FALLBACK = '0x0';
+
 export function useHasInsufficientBalance(): {
   hasInsufficientBalance: boolean;
   nativeCurrency?: string;
@@ -19,12 +21,12 @@ export function useHasInsufficientBalance(): {
   const {
     id: transactionId,
     chainId,
-    txParams: { value = '0x0', from: fromAddress = '' } = {},
+    txParams: { value = ZERO_HEX_FALLBACK, from: fromAddress = '' } = {},
   } = currentConfirmation ?? {};
 
   const batchTransactionValues =
     currentConfirmation?.nestedTransactions?.map(
-      (trxn) => (trxn.value as Hex) ?? 0x0,
+      (trxn) => (trxn.value as Hex) ?? ZERO_HEX_FALLBACK,
     ) ?? [];
 
   const chainBalances = useSelector((state) =>
@@ -34,7 +36,7 @@ export function useHasInsufficientBalance(): {
     ),
   ) as Record<Hex, Hex>;
 
-  const balance = chainBalances?.[chainId as Hex] ?? '0x0';
+  const balance = chainBalances?.[chainId as Hex] ?? ZERO_HEX_FALLBACK;
 
   const totalValue = sumHexes(value, ...batchTransactionValues);