[-] BO : fix left and right column display

Hi,
The problem is :
For pages that doesn't exists in 'ps_theme_meta' tables ( like front controller in new module, or add in override/controllers ),
$colums['left_column'] and $colums['right_column'] value is null, so the columns aren't displayed even if $this->display_column_left or $this->display_column_right is set to true.

$column test resolve this problem.
Thanks
Merge pull request #1741 from ccauw/patch-19

[-] BO : fix left and right column display