fix(enter amount): switching empty amounts should remain empty (#6297)

### Description

As the title

### Test plan

Before:
https://valora-app.slack.com/archives/C04B61SJ6DS/p1733247536281249?thread_ts=1733245811.844689&cid=C04B61SJ6DS

After:
https://github.com/user-attachments/assets/5108ab1f-73d7-4941-9ad0-17bf338dd302

### Related issues

- Related to RET-1271

### Backwards compatibility

Y
### Network scalability

If a new NetworkId and/or Network are added in the future, the changes
in this PR will:

- [ ] Continue to work without code changes, OR trigger a compilation
error (guaranteeing we find it when a new network is added)

src/components/TokenEnterAmount.test.tsx

@@ -257,6 +257,11 @@ describe('TokenEnterAmount', () => {
 
       await act(() => result.current.handleToggleAmountType())
       expect(result.current.amountType).toBe('local')
+      // the processedAmounts should be unchanged when toggling amount type with no amount entered
+      expect(result.current.processedAmounts).toStrictEqual({
+        token: { bignum: null, displayAmount: '' },
+        local: { bignum: null, displayAmount: '' },
+      })
     })
   })
 

src/components/TokenEnterAmount.tsx

@@ -179,7 +179,8 @@ export function useEnterAmount(props: {
             .replace(new RegExp(`[${decimalSeparator}]?0+$`), '')
         : ''
 
-    const parsedTokenAmount = parseInputAmount(convertedLocalToToken, decimalSeparator)
+    const parsedTokenAmount =
+      amount === '' ? null : parseInputAmount(convertedLocalToToken, decimalSeparator)
     const balanceInLocal = convertTokenToLocalAmount({
       tokenAmount: props.token.balance,
       tokenInfo: props.token,