First character of mandatory LineEdit field with JSON wizard can't be deleted

When adding a LineEdit field with mandatory flag set to "true" the very first entered character within the LineEdit can't be deleted.

Steps to reproduce:

1. Create a new JSON-based project wizard
2. Add a new page with typeID "Fields" and add a field of the form:

{ "name": "DisplayName", "trDisplayName": "Display name:", "mandatory": true, "type": "LineEdit" }

3. Run the new wizard, enter the page
4. The field is empty, the "Continue" button is disabled
5. Enter any character into the field (the "Continue" button is enabled)
6. Try to delete the entered character with backspace

Actual Result:

The entered character is still within the line edit

Expected Result:

The line edit is empty and the "Continue" button is disabled again.

(A sample wizard can be provided if needed)