[BUG][JAVA][SPRING] openapi-generator-maven-plugin can`t handle specs with nullable maps

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

When generating an API with the openapi-generator-maven-plugin based on Spring, an error occurs when using a nullable map in the API Spec.

#####OpenAPI Generator version

OpenAPI declaration file content or url

Generation Details

• generator: openapi-generator-maven-plugin
• version: 5.1.1
• generatorName: spring
• libary: spring boot

Steps to reproduce

I isolated the problem and created a simple example project: https://github.com/fweinhold/openapi-generator-spring-nullable-map-bug

The map creation in the generated put[Values] method is missing an enclosing Nullable.of().

Related issues/PRs

Suggest a fix

Start with the linked reproduction project and run its openapi-generator-maven-plugin setup using generatorName spring and version 5.1.1. Inspect the generated put[Values] method and compare its map creation with the nullable map behavior described in the issue. Done means nullable maps generate correctly without the reported error and the reproduction no longer shows the missing wrapper.