Fix invalid array key notice

When this plugin is installed and the theme of the wordpress site is
wrapped around an external system using wordpress' get_header and
get_footer methods the following notices were being thrown:

Notice: Undefined index: thickbox_js in c:\path-to-my-wordpress\wp-content\plugins\cleaner-gallery\inc\default-filters.php on line 56

Notice: Undefined index: thickbox_css in c:\path-to-my-wordpress\wp-content\plugins\cleaner-gallery\inc\default-filters.php on line 56
Ignore PHPStorm project files