bug(Luhn Checksum): invalid input cause an unhandled Character: X is not valid in radix N.

To reproduce, that drag Luhn CheckSum and set radix to 4 and then input to o or whatever invalid input
as you can see an unhandled thrown look like that

```js
Luhn Checksum - Error in blob:https://gchq.github.io/8a7d4b37-1964-4435-9bed-004d1c2a45ae on line 2.

Message: Character: o is not valid in radix 4.
```

Direct Test Links:
- https://gchq.github.io/CyberChef/#recipe=Luhn_Checksum(4)&input=bw&oeol=FF

Start with the linked CyberChef recipe and reproduce the Luhn Checksum failure using radix 4 and invalid input. Trace the Luhn Checksum operation's invalid-character handling and verify that the same input no longer produces an unhandled exception.

Written by the indexing model from the issue text.