Add filter to hide empty fields

Hello,
I’m trying to hide empty fields from the email by adding an add filter function in the functions.php. But it doesn’t seems to work.
Can someone help me on this?

add_filter( 'wpcf7_posted_data', 'hideEmptyFields' );
function hideEmptyFields( $posted_data ) {

    if( isset( $posted_data['trainee-firstname3'] ) && '' == $posted_data['trainee-firstname3'] )
        unset( $posted_data['trainee-firstname3'] );

    if( $posted_data['trainee-name3'] == '' )
        unset( $posted_data['trainee-name3'] );

    return $posted_data;
}

Simply need to hide as many empty fields as I have:
[trainee-firstnameX] and [trainee-nameX]

Thx for your help,
N