refactor: cosmetic changes (#6123)

app/Http/Controllers/DAV/Backend/CalDAV/CalDAVBackend.php

@@ -176,7 +176,7 @@ class CalDAVBackend extends AbstractBackend implements SyncSupport
                     return $backend->prepareData($date);
                 })
                 ->filter(function ($event) {
-                    return ! is_null($event);
+                    return $event !== null;
                 })
                 ->toArray();
         }

app/Http/Controllers/DAV/Backend/CardDAV/AddressBookHome.php

@@ -38,7 +38,7 @@ class AddressBookHome extends BaseAddressBookHome
     {
         $addressBooks = $this->carddavBackend->getAddressBooksForUser($this->principalUri);
 
-        return collect($addressBooks)->map(function ($addressBook) {
+        return collect($addressBooks)->map(function (array $addressBook): AddressBook {
             return new AddressBook($this->carddavBackend, $addressBook);
         })->toArray();
     }

app/Http/Controllers/DAV/Backend/SyncDAVBackend.php

@@ -92,7 +92,10 @@ trait SyncDAVBackend
     public function getLastModified($collectionId)
     {
         return $this->getObjects($collectionId)
-                    ->max('updated_at');
+                    ->map(function ($object) {
+                        return $object->updated_at;
+                    })
+                    ->max();
     }
 
     /**