[BUG][JAVA] Java-Jersey2 generator creates "Accept:null" header

Description

I use Jersey2 as generator for my REST, including DELETE operations that don't "accept" any body.
As a result, I see that the generated code executes HTTP requests with header "Accept:null" that is not a valid header (because of "null" string).

openapi-generator version

I use openapi-generator-maven-plugin 4.2.3, but from what I see in source code this bug should be reproducible in the master too.

OpenAPI declaration file content or url

paths:
  /v2/something/{name}:
    delete:
      tags:
      - Repository
      summary: Delete something
      operationId: something_delete
      parameters:
      - name: paramone
        in: path
        required: true
        schema:
          type: string
      responses:
        "204":
          description: delete success or nothing to delete
        "401":
          description: not authenticated / invalid credentials
        "403":
          description: not authorized, the current user does not have the "repository"
            privilege
        "500":
          description: internal server error

Command line used for generation

pom.xml config:

                <groupId>org.openapitools</groupId>
                <artifactId>openapi-generator-maven-plugin</artifactId>
                <version>4.2.3</version>
...
                            <generatorName>java</generatorName>
                            <configOptions>
                                <java8>true</java8>
                                <library>jersey2</library>
                            </configOptions>
...

Steps to reproduce

See description, it's quite obvious.

Related issues/PRs

Not found

Suggest a fix

In file openapi-generator/src/main/resources/Java/libraries/jersey2/ApiClient.mustache change line from

Invocation.Builder invocationBuilder = target.request().accept(accept);

to

Invocation.Builder invocationBuilder = accept != null ? target.request().accept(accept) : target.request();

Also, need to check other generator for this problem.

Start in openapi-generator/src/main/resources/Java/libraries/jersey2/ApiClient.mustache and inspect how the Jersey2 client builds request headers for operations without response bodies. Generate the provided DELETE operation with the Java generator and verify that the resulting request does not send an Accept:null header; also check whether other generators contain the same pattern.