Invalid characters from a MaskFormatter

So, guys, I have a window where I need to pass a validation of null fields of several JTextField and a JFormattedTextField , where the latter has a mask for phone numbers "(##)####-####" . The fact is that even though I do not type anything in this field, it takes the characters from the mask: ()- and understands that the field is not null. I would like to know if there is any method that ignores characters from a mask, or will I have to do all that trap with substring ? It is worth mentioning that I just want to ignore it at the time of the validation, and do not remove those characters, because then I will have to introduce them to the user!

How to instantiate:

try {
        mskTel = new MaskFormatter("(##)####-####");
    } catch (ParseException e) {
        e.printStackTrace();
    }

    txtTel = new JFormattedTextField(mskTel);

How the validation is done:

form.txtTel.getText() == null || form.txtTel.getText().trim().isEmpty()