forgot case for previewPicture filter

src/Wallabag/CoreBundle/Filter/EntryFilterType.php

@@ -45,6 +45,12 @@ class EntryFilterType extends AbstractType
             ->add('isStarred', 'filter_checkbox')
             ->add('previewPicture', 'filter_checkbox', array(
                 'apply_filter' => function (QueryInterface $filterQuery, $field, $values) {
+                    $value = $values['value'];
+
+                    if (false === $value) {
+                        return;
+                    }
+
                     $expression = $filterQuery->getExpr()->isNotNull($field);
 
                     return $filterQuery->createCondition($expression);