empty（func（$ var））触发“无法使用函数返回值”错误

This code:

if(  get_theme_mod('enable_content_background_color') && empty( get_theme_mod('content_background_image') ) ) {
         $custom .= " .site-content {
               background-image: none;
          }"."
";

Triggers this error:

Can't use function return value in write context in /home3/*******/public_html/*******/wp-content/themes/outliner/includes/styles.php on line 41

I wonder why?

The issue here is that the empty() function only supports variables. If you look at the PHP.net Doc for the empty method you'll see;

Note: Prior to PHP 5.5, empty() only supports variables; anything else will result in a parse error. In other words, the following will not work: empty(trim($name)). Instead, use trim($name) == false.

You'll need to do something like;

$contentBgImage = get_theme_mod('content_background_image');    
if( get_theme_mod('enable_content_background_color') && empty($contentBgImage ) ) { 
//code
}