[BUGFIX] SelectCategory.php: Cast $value to string (#156)

If $value is set bei "getId()" in line 73, it causes an error within "trim()" in line 73, because it's an integer not a string.

src/FilterService/FilterType/ElasticSearch/SelectCategory.php

@@ -70,7 +70,7 @@ public function addCondition(AbstractFilterDefinitionType $filterDefinition, Pro
         } elseif (empty($value) && !$isReload && method_exists($filterDefinition, 'getPreSelect')) {
             $value = $filterDefinition->getPreSelect();
             if ($value instanceof ElementInterface) {
-                $value = $value->getId();
+                $value = (string)$value->getId();
             }
         }