Add new parameter "empty" to {serendipity_showPlugin}

docs/NEWS

@@ -3,6 +3,15 @@
 Version 1.6 ()
 ------------------------------------------------------------------------
 
+    * Added new parameter "empty" to {serendipity_showPlugin}. When
+      no callable plugins were found, the string in the "empty" parameter
+      will be shown instead, allowing users to get notified of a missing
+      plugin:
+      
+       {serendipity_showPlugin 
+           class="serendipity_plugin_twitter"
+           empty="Twitter plugin not found!"}
+
     * Bundle jquery by default and enable it in frontend and backend
       templates; overrides serendipity_event_jquery. If your template
       contains its own "jquery.js" file, the core will NOT use it.

include/functions_smarty.inc.php

@@ -469,7 +469,13 @@ function serendipity_smarty_showPlugin($params, &$smarty) {
         $params['template'] = 'sidebar.tpl';
     }
 
-    return serendipity_plugin_api::generate_plugins($params['side'], null, $params['negate'], $params['class'], $params['id'], $params['template']);
+	$out = serendipity_plugin_api::generate_plugins($params['side'], null, $params['negate'], $params['class'], $params['id'], $params['template']);
+
+	if (empty($out) && !empty($params['empty'])) {
+		return $params['empty'];
+	}
+
+    return $out;
 }
 
 /**