[BUG] 6.2.1 regression - JavaSpring `@NotNull` code annotation with no import

Bug Report Checklist

• [Yes] Have you provided a full/minimal spec to reproduce the issue?
• [No] Have you validated the input using an OpenAPI validator (example)?
• [No] Have you tested with the latest master to confirm the issue still exists?
• [Yes] Have you searched for related issues/PRs?
• [Yes] What's the actual output vs expected output?

Description

JavaSpring generator creates java files with @NotNull but there's no import for that symbol, resulting in javac error cannot find symbol.

openapi-generator version

6.2.1

This is a regression, was working on 6.2.0.

OpenAPI declaration file content or url

    AuthzRequestStatus:
      type: string
      enum: ['APPROVED', 'REQUESTED', 'REJECTED']

Full file: https://github.com/au-research/raido-v2/blob/895de24f6dc9ec3a360ba04acf0f3ee1355b6d59/api-svc/idl-raid-v2/src/admin-experimental.yaml#L334

Generation Details

Project uses the Gradle plugin to generate, full configuration is here:
https://github.com/au-research/raido-v2/blob/895de24f6dc9ec3a360ba04acf0f3ee1355b6d59/api-svc/idl-raid-v2/build.gradle#L136

I think the relevant configuration item might be: openApiNullable: "false" and useBeanValidation: "false"?

Generates code like:

Steps to reproduce

• run the standard Gradle openApiGenerate task
• then compile the generated code
• javac fails with error cannot find symbol

Suggest a fix

If openApiNullable and useBeanValidation set to false - 6.2.1 should not generate the @NotNull annotation. 6.2.0 does not generate the annotation.

Workaround

Customise pojo.mustache template to effective revert the effect of commit 0060dfd1.
https://github.com/au-research/raido-v2/blob/main/api-svc/idl-raid-v2/src/main/resources/template/pojo.mustache#L3

Start with the JavaSpring generator's pojo.mustache template and the behavior introduced by commit 0060dfd1. Run the standard Gradle openApiGenerate task with openApiNullable and useBeanValidation disabled, then compile the generated Java; done means the generated code has no unresolved @NotNull annotation.