Allow id 0 in addition to dbValues

Hi,

In Editor, I have a simple join, showing a select dropdown field with some labels and storing its id number in db. The input is validated using ->validator('Validator::dbValues')

I would like to make this select field optional (not requred), so user should be able to select nothing (=placeholder). I can setup the placeholder options (incl. placeholderDisabled: false, placeholderValue: 0). But how can I allow the value 0 to be allowed for the validator in the PHP file?

Best regards,
marwi