[custom_content_issue] (#12836)

This PR will fix issue: https://github.com/OpenAPITools/openapi-generator/issues/12835

Signed-off-by: the-akhil-nair <sam1232007@gmail.com>

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

@@ -171,7 +171,7 @@ public class ExampleGenerator {
                         output.add(kv);
                     }
                 } else {
-                    kv.put(EXAMPLE, null);
+                    kv.put(EXAMPLE, "null");
                     output.add(kv);
                 }
             }