[SPRING] implicitHeaders crashes if all parameters on an operation are header parameters

Description

If implicitHeaders=true is set when generating an API for a Spring server and an operation contains only header parameters, the code crashes due to an out of bounds exception.

Exception in thread "Thread-0" java.lang.RuntimeException: Could not generate api file for 'div'
	at io.swagger.codegen.v3.DefaultGenerator.generateApis(DefaultGenerator.java:569)
	at io.swagger.codegen.v3.DefaultGenerator.generate(DefaultGenerator.java:776)
	at io.swagger.codegen.v3.cli.cmd.Generate.run(Generate.java:342)
	at java.base/java.lang.Thread.run(Thread.java:834)
Caused by: java.lang.IndexOutOfBoundsException: Index -1 out of bounds for length 0
	at java.base/jdk.internal.util.Preconditions.outOfBounds(Preconditions.java:64)
	at java.base/jdk.internal.util.Preconditions.outOfBoundsCheckIndex(Preconditions.java:70)
	at java.base/jdk.internal.util.Preconditions.checkIndex(Preconditions.java:248)
	at java.base/java.util.Objects.checkIndex(Objects.java:372)
	at java.base/java.util.ArrayList.get(ArrayList.java:458)
	at io.swagger.codegen.v3.generators.java.SpringCodegen.removeHeadersFromAllParams(SpringCodegen.java:570)
	at io.swagger.codegen.v3.generators.java.SpringCodegen.postProcessOperations(SpringCodegen.java:509)
	at io.swagger.codegen.v3.DefaultGenerator.processOperations(DefaultGenerator.java:984)
	at io.swagger.codegen.v3.DefaultGenerator.generateApis(DefaultGenerator.java:486)
	... 3 more

Swagger-codegen version

3.0.10.

Swagger declaration file content or url

openapi: 3.0.0
info:
  title: Failed Header Demo
  version: 1.0.0
  description: Demonstrate a crash in Swagger Codegen 3.0.10
paths:
  /div:
    get:
      tags:
        - div
      operationId: getV1Div
      parameters:
        - in: header
          name: My-Request-Id
          description: Unique Correlation Id For Each Request
          required: true
          schema:
            type: string
      responses:
        200:
          description: Successful Response
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/DivResponse'
components:
  schemas:
    DivResponse:
      type: object
      properties:
        divs:
          type: array
          items:
            type: string
      required:
        - divs

Command line used for generation

swagger-codegen generate -i bug.yml
-l spring
--api-package com.foo.api
-Dapis=div
--additional-properties implicitHeaders=true

Steps to reproduce

1. Execute the command above with the provided file.
2. Witness the crash in all its glory.

Related issues/PRs

None

Suggest a fix/enhancement

The offending line of code is here. The bug in the algorithm is obvious from a simple code review. The allParams parameter is duplicated and then cleared, so it has a size of 0. Non-header parameters from the copy of the parameters are added back into allParams. Because of this filtering, it is possible for allParams to still have a size of 0. And then, the last item in the list is modified by using the size of the list minus 1 to identify the index. This results in a call to List<>.get(-1), which is clearly invalid.

Start with SpringCodegen.java, specifically removeHeadersFromAllParams, and reproduce the failure using the supplied OpenAPI document and generation command. The work is done when Spring server generation succeeds for an operation whose only parameter is a header and the generated API is produced without the IndexOutOfBoundsException.