fix top-level enum class checking (OpenAPITools#1120)

Author: phstudy

modules/openapi-generator/src/main/java/org/openapitools/codegen/utils/ModelUtils.java

@@ -652,7 +652,7 @@ public static Schema unaliasSchema(Map<String, Schema> allSchemas, Schema schema
             if (ref == null) {
                 LOGGER.warn("{} is not defined", schema.get$ref());
                 return schema;
-            } else if (isStringSchema(ref) && (ref.getEnum() != null && !ref.getEnum().isEmpty())) {
+            } else if (ref.getEnum() != null && !ref.getEnum().isEmpty()) {
                 // top-level enum class
                 return schema;
             } else if (isArraySchema(ref) || isComposedSchema(ref)) { // array def should be created as models