[BUG][Spring] can't use `pattern` for the `nullable` property in request body

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

If the request body definition contains properties with both nullable and pattern, the generated Spring stub server will throw an exception when the request is received.

openapi-generator version

$ openapi-generator-cli version
4.0.3-SNAPSHOT

$ git log --oneline -n 1
51e7005373 [online] Remove GENERATOR_HOST defaults (#3289)

OpenAPI declaration file content or url

See: https://gist.github.com/autopp/ddb42bee720ba0147247da81b6647138

Note that the property foo in the request body for /hello is specified both nullable: true and pattern.

                foo:
                  type: string
                  nullable: true
                  pattern: '[a-z]+'

Command line used for generation

$ java -jar modules/openapi-generator-cli/target/openapi-generator-cli.jar generate --skip-validate-spec -g spring -o /tmp/spring -i example.yml --additional-properties returnSuccessCode=true

Steps to reproduce

1. Run the above command to generate Spring server.
2. Build and run generated server.
3. Request to /hello with payload

   $ curl -X POST localhost:8080/hello -H 'Content-Type: application/json' -d '{"foo":"abc"}'
   

The client should get the error response:

{"timestamp":"2019-07-08T00:52:28.547Z","status":500,"error":"Internal Server Error","message":"HV000030: No validator could be found for constraint 'javax.validation.constraints.Pattern' validating type 'org.openapitools.jackson.nullable.JsonNullable<java.lang.String>'. Check configuration for 'foo'","path":"/hello"}

And server output error log:

2019-07-08 09:52:22.719 ERROR 66621 --- [nio-8080-exec-1] o.a.c.c.C.[.[.[/].[dispatcherServlet]    : Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is javax.validation.UnexpectedTypeException: HV000030: No validator could be found for constraint 'javax.validation.constraints.Pattern' validating type 'org.openapitools.jackson.nullable.JsonNullable<java.lang.String>'. Check configuration for 'foo'] with root cause

javax.validation.UnexpectedTypeException: HV000030: No validator could be found for constraint 'javax.validation.constraints.Pattern' validating type 'org.openapitools.jackson.nullable.JsonNullable<java.lang.String>'. Check configuration for 'foo'
...

Whole log: https://gist.github.com/autopp/c58955e0277b7dbb3c85bbffcfa89a6c

In my use case, I want to use both nullable and pattern simultaneously.

Related issues/PRs

N/A

Suggest a fix

Sorry, I have no idea.

Start by reproducing the generated Spring server with the command in modules/openapi-generator-cli and the nullable string property using both pattern and nullable. Trace the Spring generator output and validation handling for that property. Done means a valid request such as {"foo":"abc"} no longer raises the Pattern validator exception while the pattern constraint remains effective.