Fix Engine: default not leading to default templates being used

Author: onli

docs/NEWS

@@ -1,6 +1,9 @@
 Version 2.6-alpha1 ()
 ------------------------------------------------------------------------
 
+   * Fix option to set a different theme as fallback option by setting
+     Engine: $themename in a themes info.txt
+
    * Let installer check for the intl extension, needed for the strftime
      compatibility library used for PHP 8.1+
 

include/functions_config.inc.php

@@ -266,7 +266,7 @@ function serendipity_getTemplateFile($file, $key = 'serendipityHTTPPath', $force
     } else {
         $directories[] = $serendipity['template_backend'] . '/';    # Since 2.0 s9y can have a independent backend theme
     }
-    $directories[] = $serendipity['template_engine'] ?? null . '/'; # themes can set an engine, which will be used if they do not have the file
+    $directories[] = ($serendipity['template_engine'] ?? null) . '/'; # themes can set an engine, which will be used if they do not have the file
     $directories[] = $serendipity['defaultTemplate'] .'/';  # the default theme is the last place we will look in, serving as pure fallback
     $directories = array_unique($directories); # save performance by not checking for file existence multiple times in the same directory