fix: render null values included in enum (#9683)

Co-authored-by: Vladimír Gorej <vladimir.gorej@gmail.com>

src/core/plugins/json-schema-5/components/enum-model.jsx

@@ -7,7 +7,7 @@ const EnumModel = ({ value, getComponent }) => {
   return <span className="prop-enum">
     Enum:<br />
     <ModelCollapse collapsedContent={ collapsedContent }>
-      [ { value.join(", ") } ]
+      [ { value.map(String).join(", ") } ]
     </ModelCollapse>
   </span>
 }
@@ -16,4 +16,4 @@ EnumModel.propTypes = {
   getComponent: ImPropTypes.func
 }
 
-export default EnumModel
+export default EnumModel