[bug] Fix - Email copy to own address doesn't work since 2.5.21
Related to bug - [bug] [33848] Fix fatal error when sending contact form

If statement optimization - use isset instead of !empty because the request variable is empty if the input field has no value parameter even though the checkbox is activated
from : https://github.com/Kubik-Rubik/joomla-cms/commit/0d851c03f92d16c3fc2d36b5fb374e067f59468e
Merge branch '2.5.21'

* 2.5.21:
  [bug] Fix - Email copy to own address doesn't work since 2.5.21 Related to bug - [bug] [33848] Fix fatal error when sending contact form