Fix #3753 no available themes

This is the minimal fix, just a better message. It would be nice to handle this better in upgrades to 7.9 where SuiteP is disabled.

include/SugarTheme/SugarTheme.php

@@ -1423,6 +1423,9 @@ public static function availableThemes()
             $themelist[$themeobject->dirName] = $themeobject->name;
         }
         asort($themelist, SORT_STRING);
+        if (count($themelist)==0) {
+		$GLOBALS['log']->fatal("availableThemes() is returning empty array! Check disabled_themes in config.php and config_override.php");
+        }
         return $themelist;
     }