Bug: Cannot remove custom font once a valid URL is set

[camiicode]

Problem:
After adding a custom Google Fonts URL in the custom font input,
the user can save the font successfully.

However, if the user tries to remove the custom font by clearing
the input field, the app prevents saving and requires the input
to contain a valid Google Fonts URL again.

This makes it impossible to revert to the default font.

Expected behavior:
Users should be able to clear the custom font input
and restore the default font without validation errors.

Possible cause:
The input validation logic does not allow empty values
and assumes a URL is always required.

Steps to reproduce:

1. Open font settings.
2. Add a valid Google Fonts URL.
3. Save changes successfully.
4. Clear the custom font input.
5. Attempt to save again.
6. Observe validation blocking the action.

Acceptance criteria:

• Empty input is allowed.
• Clearing the input restores the default font.
• No validation error when input is empty.