[BUG] unexpected generated code when override security scheme on swagger2.0

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

I encountered a problem that a unexpected code was generated when override security schema on swagger2.0.
In the yaml example below, the authentication code for both APIs were generated.

Also, I checked a variable authMethods in template. And its value was unexpected.

openapi-generator version

openapi-generator-cli 5.0.0-SNAPSHOT
  commit : 54b37c6
  built  : 2020-08-03T07:20:03Z
  source : https://github.com/openapitools/openapi-generator
  docs   : https://openapi-generator.tech/

OpenAPI declaration file content or url

swagger: "2.0"
info:
  title: title
  version: 0.0.1
  description: description
securityDefinitions:
  Token:
    type: apiKey
    in: header
    name: Authorization
security:
  - Token: []

paths:
  /user:
    get:
      responses:
        200:
          description: description
  /signin:
    get:
      security: []
      responses:
        200:
          description: description

Generation Details

docker run --rm  -v ${PWD}:/local openapitools/openapi-generator-cli:latest generate \
		--generator-name php \
		--input-spec /local/openapi.yaml \
		--output /local/output_php

Steps to reproduce

same as Generation Details

Related issues/PRs

none

Suggest a fix

none

Start with /local/openapi.yaml and the PHP generation command, then inspect how the template's authMethods handles the operation with security: []. Done means /signin generates no authentication code while /user still does; verify the generated output against the reported behavior and add a regression test if an existing generator test entry point is found.