BHV-14452: CheckboxItem doens't call srcChanged/iconChanged on initial. . .

. . .render when it is set to empty string

### Issue:
When user wants to set icon and src to "" to clear icon, icon changed is not called on render.
### Fix:
use this.hasOwnProperty('icon') instead of this.icon

Enyo-DCO-1.1-Signed-off-by: Krishna Rangarajan <krishna.rangarajan@lge.com>