Fixed notice

Civi/FormProcessor/Type/OptionGroupType.php

@@ -162,7 +162,7 @@ class OptionGroupType extends AbstractType implements OptionListInterface {
       }
       return $return;
     }
-    elseif (!is_array($value)) {
+    elseif (!is_array($value) && strlen($value)) {
       return $this->denormalizedOptions[$value];
     }
     else {