The URL field on smartcards cannot be cleared (it works for the name field).
To reproduce:
1. Open Smartcards Dialog (in Kleopatra)
2. Add an URL
3. Edit the URL, clear the text, OK => Error
{F31156369}
Kleopatra simply sends `SCD SETATTR PUBKEY-URL ` to gpg-agent which fails with "Card error" (code: 108, source: SCD).