fix: Index out of range in examples/credit-card-form when ccn is empty

[grafviktor]

Fixes Index out of range error in examples/credit-card-form when credit card number(CCN) is not provided.

To reproduce:

• Run examples/credit-card-form/main.go
• Do not type anything in "Card number"
• Press UP or DOWN keys on your keboard (or press TAB several times)

ccnValidator will trigger and will attempt to read a symbol which is located at len("")-1, that will cause Index out of range error (because CCN is empty).

[maaslalani]

Thanks so much!