Fix the URL generation for effectively empty query strings

`\http_build_query()` explicitly discards any component that has a `NULL` value. This can lead to an empty string being returned which was not handled previously.

Author: BurntimeX

wcfsetup/install/files/lib/system/request/route/DynamicRequestRoute.class.php

@@ -230,8 +230,11 @@ protected function buildRoute(array $components, $application, $useBuildSchema)
         }
 
         if ($components !== []) {
-            $link .= \str_contains($link, '?') ? '&' : '?';
-            $link .= \http_build_query($components, '', '&');
+            $queryString = \http_build_query($components, '', '&');
+            if ($queryString !== '') {
+                $link .= \str_contains($link, '?') ? '&' : '?';
+                $link .= $queryString;
+            }
         }
 
         return $link;