Clean up TokenAllowance customPermissionAmount prop. It should be 1 type instead of string or number

### Describe the bug

Currently in the TokenAllowance component, the `customPermissionAmount` prop may be set to the value 0 as a string or a number. We should expect a consistent type.,

### Steps to reproduce

1. https://metamask.github.io/test-dapp/
2. Send Tokens > Create Tokens > Approve Tokens
3. Inspect `customPermissionAmount` variable

### Error messages or log output

_No response_

### Version

11.0

### Build type

None

### Browser

Chrome

### Operating system

MacOS

### Hardware wallet

_No response_

### Additional context

_No response_

Locate the TokenAllowance component and search its usages for customPermissionAmount, then inspect the Approve Tokens flow in the linked test dapp. Done means the prop has one consistent type rather than accepting both string and number values, with any relevant checks or tests updated accordingly.

Written by the indexing model from the issue text.