Validators don't validate the value which will be stored (empty string vs null)

Steps to reproduce

1.  Define a field like so:

 twitter:
          required: false
          $type: textField
          validators:
            - name: regexp
              pattern: '[a-zA-Z0-9_]{1,15}'
              $type: regexpValidator

Fill in the empty string. The field is not required. It will not validate.

If I remove the validator, it will store, but not fill in the field. The empty string is interpreted as null.

But the regexpvalidator would have accepted null too.

This particular case can be easily worked around. But how about $type: emailValidator?