CI: Fix psalm NullReference errors

public/main/auth/my_progress.php

@@ -25,7 +25,7 @@ $pluginCalendar = 'true' === api_get_plugin_setting('learning_calendar', 'enable
 
 if ($pluginCalendar) {
     $plugin = LearningCalendarPlugin::create();
-    $plugin->setJavaScript($htmlHeadXtra);
+    $plugin->setJavaScript();
 }
 
 $user_id = api_get_user_id();

public/main/my_space/myStudents.php

@@ -711,7 +711,7 @@ $pluginCalendar = 'true' === api_get_plugin_setting('learning_calendar', 'enable
 
 if ($pluginCalendar) {
     $plugin = LearningCalendarPlugin::create();
-    $plugin->setJavaScript($htmlHeadXtra);
+    $plugin->setJavaScript();
 }
 
 Display::display_header($nameTools);

public/plugin/learning_calendar/LearningCalendarPlugin.php

@@ -798,11 +798,10 @@ class LearningCalendarPlugin extends Plugin
         return 0;
     }
 
-    /**
-     * @param array $htmlHeadXtra
-     */
-    public function setJavaScript(&$htmlHeadXtra)
+    public function setJavaScript()
     {
+        global $htmlHeadXtra;
+
         $htmlHeadXtra[] = api_get_js('jqplot/jquery.jqplot.js');
         $htmlHeadXtra[] = api_get_js('jqplot/plugins/jqplot.dateAxisRenderer.js');
         $htmlHeadXtra[] = api_get_js('jqplot/plugins/jqplot.canvasOverlay.js');