[bug] Fix - Email copy to own address doesn't work since 2.5.21

Related to 
[bug] [33848] Fix fatal error when sending contact form (https://github.com/joomla/joomla-cms/pull/3763)

**How to test**

- Use Joomla! version 2.5.22 and create a contact - set a link to the contact with a form in the menu
- Fill out the form with some sample data (but correct email address), activate the checkbox "Send copy to yourself" and click on send
- You will never receive a copy of this email because the value of this checkbox input field is empty
- Add patch and try it again