code: def non_exist_username(app):
      username = "fake_username"
      user = app.appbuilder.sm.find_user(username)
      if user is not None:
          app.appbuilder.sm.del_register_user(user)
      yield username
      user = app.appbuilder.sm.find_user(username)
      if user is not None:
          app.appbuilder.sm.del_register_user(user)
  
imports: from __future__ import annotations
  import datetime
  import json
  import pytest
  from airflow.jobs.job import Job
  from airflow.jobs.scheduler_job_runner import SchedulerJobRunner
  from airflow.utils import timezone
  from airflow.utils.session import create_session
  from airflow.www import app
  from tests.test_utils.asserts import assert_queries_count
  from tests.test_utils.config import conf_vars
  from tests.test_utils.www import check_content_in_response, check_content_not_in_response
label_desc: Use image node_id 29 for calling a global function with example usage: non_exist_username(app) without return types