Custom executor config added when registering a new executor

Overview

Running the command below will register a new docker executor

$ gitlab-runner register -n \
    --url https://gitlab.com/ \
    --registration-token xxxx \
    --executor docker \
    --description "My Docker Runner" \
    --docker-image "docker:stable" \

Which will generate the following config.toml file

[[runners]]
  name = "My Docker Runner"
  url = "https://gitlab.com/"
  token = "EftCwBhsCHSsJh53YMsw"
  executor = "docker"
  [runners.custom_build_dir]
  [runners.docker]
    tls_verify = false
    image = "docker:stable"
    privileged = false
    disable_entrypoint_overwrite = false
    oom_kill_disable = false
    disable_cache = false
    volumes = ["/cache"]
    shm_size = 0
  [runners.cache]
    [runners.cache.s3]
    [runners.cache.gcs]
  [runners.custom]
    run_exec = ""

Notice how it creates [runners.custom] section, which is for another executor and it's not needed. That section should only be populated when the custom executor is choosen.