[#19439] Don't assume schema is null type if ref is not null in OpenAPINormalizer#isNullTypeSchema

Author: beikov

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

@@ -912,7 +912,7 @@ public boolean isNullTypeSchema(Schema schema) {
         }
 
         if (schema instanceof JsonSchema) { // 3.1 spec
-            if (Boolean.TRUE.equals(schema.getNullable())) {
+            if (Boolean.TRUE.equals(schema.getNullable()) && schema.get$ref() == null) {
                 return true;
             }