Allow empty standard layers.

src/Listener/Dca/MapDcaListener.php

@@ -15,6 +15,7 @@ declare(strict_types=1);
 namespace Netzmacht\Contao\Leaflet\Listener\Dca;
 
 use Contao\DataContainer;
+use Contao\StringUtil;
 use Doctrine\DBAL\Connection;
 use Netzmacht\Contao\Leaflet\Model\LayerModel;
 use Netzmacht\Contao\Toolkit\Dca\Listener\AbstractListener;
@@ -88,7 +89,7 @@ class MapDcaListener extends AbstractListener
      */
     public function saveLayerRelations($layerId, $dataContainer)
     {
-        $new       = deserialize($layerId, true);
+        $new       = array_filter(StringUtil::deserialize($layerId, true));
         $values    = array();
         $statement = $this->connection->prepare('SELECT * FROM tl_leaflet_map_layer WHERE mid=:mid order BY sorting');
 

src/Resources/contao/dca/tl_leaflet_map.php

@@ -224,7 +224,7 @@ $GLOBALS['TL_DCA']['tl_leaflet_map'] = array
                         'inputType'        => 'select',
                         'options_callback' => ['netzmacht.contao_leaflet_maps.listeners.dca.map', 'getLayers'],
                         'eval'             => array(
-                            'mandatory'          => true,
+                            'mandatory'          => false,
                             'tl_class'           => 'w50',
                             'chosen'             => true,
                             'includeBlankOption' => true,