prevent php8-warnings, closes #3169

Closes #3169

Merge request studip/studip!2146

app/views/resources/room_planning/booking_plan.php

@@ -46,10 +46,10 @@
         _('Belegungsplan'),
         [
             'editable' => true,
-            'selectable' => ($fullcalendar_studip_urls['add'] != null),
+            'selectable' => !empty($fullcalendar_studip_urls['add']),
             'studip_urls' => $fullcalendar_studip_urls,
-            'minTime' => ($min_time),
-            'maxTime' => ($max_time),
+            'minTime' => $min_time,
+            'maxTime' => $max_time,
             'allDaySlot' => false,
             'header' => [
                 'left' => 'dayGridMonth,timeGridWeek,timeGridDay',