_isEmpty(modelValue = this.modelValue) {
    // the activeRegion is not synced on time, so it can't be used in this check
    return super._isEmpty(modelValue) || this.value === this.__initializedRegionCode;
  }
Used for Required validation and computation of interaction states.
We need to override this, because we prefill the input with the region code (like +31), but for proper UX,
we don't consider this as having interaction state `prefilled`
@param {string} modelValue
@return {boolean}
@protected