fix layout changes since new hook `nav_menu` (#3362)

* fix layout if no hooks `nav_menu` are active

* Update app/layout/nav_menu.phtml

oh thx

Co-authored-by: Alexandre Alapetite <alexandre@alapetite.fr>

Co-authored-by: Alexandre Alapetite <alexandre@alapetite.fr>

app/layout/nav_menu.phtml

@@ -175,9 +175,12 @@
 		</a>
 	</div>
 
+	<?php $nav_menu_hooks = Minz_ExtensionManager::callHook('nav_menu'); ?>
+	<?php if ($nav_menu_hooks != '') { ?>
 	<div class="stick" id="nav_menu_hooks">
-		<?= Minz_ExtensionManager::callHook('nav_menu') ?>
+		<?php echo $nav_menu_hooks; ?>
 	</div>
+	<?php } ?>
 
 	<div class="item search">
 		<form action="<?= _url('index', 'index') ?>" method="get">