Metric collector is failing when meet with DagRun in "empty" state

[KulykDmytro]

[2021-09-03 12:52:35,747] {app.py:1891} ERROR - Exception on /admin/metrics/ [GET]
Traceback (most recent call last):
  File "/home/airflow/.local/lib/python3.8/site-packages/flask/app.py", line 2447, in wsgi_app
    response = self.full_dispatch_request()
  File "/home/airflow/.local/lib/python3.8/site-packages/flask/app.py", line 1952, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/home/airflow/.local/lib/python3.8/site-packages/flask/app.py", line 1821, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/home/airflow/.local/lib/python3.8/site-packages/flask/_compat.py", line 39, in reraise
    raise value
  File "/home/airflow/.local/lib/python3.8/site-packages/flask/app.py", line 1950, in full_dispatch_request
    rv = self.dispatch_request()
  File "/home/airflow/.local/lib/python3.8/site-packages/flask/app.py", line 1936, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/home/airflow/.local/lib/python3.8/site-packages/airflow_prometheus_exporter/prometheus_exporter.py", line 492, in list
    return Response(generate_latest(), mimetype='text')
  File "/home/airflow/.local/lib/python3.8/site-packages/prometheus_client/exposition.py", line 137, in generate_latest
    output.append(sample_line(s))
  File "/home/airflow/.local/lib/python3.8/site-packages/prometheus_client/exposition.py", line 93, in sample_line
    ['{0}="{1}"'.format(
  File "/home/airflow/.local/lib/python3.8/site-packages/prometheus_client/exposition.py", line 94, in <listcomp>
    k, v.replace('\\', r'\\').replace('\n', r'\n').replace('"', r'\"'))
AttributeError: ("'NoneType' object has no attribute 'replace'", Metric(airflow_dag_status, Shows the number of dag starts with this status, gauge, , [Sample(name='airflow_dag_status', labels={'dag_id': 'adpool_hourly', 'owner': 'airflow', 'status': 'failed'}, value=16, timestamp=None, exemplar=None), ...]))