N°6307 Fix alert when caselog filled and submitting form

Regression was introduced in 8dc7d68b
Temporary fix, as the alert is caused by activity-panel.js which is disabled only if another handler is registered before 🤯 As a temporary fix I add an empty handler at the same place as the one that was there before, but we need a better way to disable activity-panel.js handler !

application/cmdbabstract.class.inc.php

@@ -2955,6 +2955,9 @@ JS
 		$oPage->add_ready_script(<<<JS
 // Try to release concurrent lock when leaving the page
 $(window).on('unload',function() { return OnUnload('$iTransactionId', '$sClass', $iKey, $sJSToken) } );
+window.onbeforeunload = function() {
+	// N°6307 just an empty function to disable activity-panel.js handler
+};
 
 // Leave handler for the current form (check if in a modal or not)
 // Note: We use a self-invoking function to avoid making unique vars. names to avoid collision (this can be called multiple time if modal forms are displayed)