OpenAPI Generator team implemented jackson-databind-nullable module which you should include to your project. The newest version is 0.2.6.

<dependency>
    <groupId>org.openapitools</groupId>
    <artifactId>jackson-databind-nullable</artifactId>
    <version>0.2.6</version>
</dependency>

If module will not be detected automatically you need to do it manually by:

ObjectMapper mapper = new ObjectMapper();
mapper.registerModule(new JsonNullableModule());

Or if you using Jackson in Spring project, then you can register it by:

@Bean
@Primary
public Jackson2ObjectMapperBuilder customObjectMapper() {
    return new Jackson2ObjectMapperBuilder()
            // other configs are possible
            .modules(new JsonNullableModule());
}