exec_run does not produce exit_code when both stderr and stdout are False

See below for an example of this behaviour. Essentially, I would like to be able to run a code, ignore its output, and just check its exit code. A workaround is to redirect the stderr/stdout to /dev/null in this case (i.e., "echo 'hello world' > /dev/null 2> /dev/null), but I would expect docker-py to be able to handle this use case without the need for manual instrumentation.

client = docker.from_env()
container = client.containers.run(
    'alpine', "sh -c 'sleep 60'", detach=True)

exec_output = \
    container.exec_run("echo 'hello world'")
print(exec_output)
# ExecResult(exit_code=0, output=b'hello world\n')

exec_output_without_stdout = \
    container.exec_run("echo 'hello world'", stdout=False)
print(exec_output_without_stdout)
# ExecResult(exit_code=0, output=b'')

exec_output_without_stderr = \
    container.exec_run("echo 'hello world'", stderr=False)
print(exec_output_without_stderr)
# ExecResult(exit_code=0, output=b'hello world\n')

exec_output_without_stdout_or_stderr = \
    container.exec_run("echo 'hello world'", stdout=False, stderr=False)
print(exec_output_without_stdout_or_stderr)
# ExecResult(exit_code=None, output=b'')

container.remove(force=True)

For reference, here are some details about my environment:

(docker-py) $ docker --version
Docker version 19.03.1-ce, build 74b1e89e8a
(docker-py) $ python --version
Python 3.7.4
(docker-py) $ pip list
Package          Version  
---------------- ---------
certifi          2019.9.11
chardet          3.0.4    
docker           4.1.0    
idna             2.8      
pip              19.3.1   
requests         2.22.0   
setuptools       41.4.0   
six              1.12.0   
urllib3          1.25.6   
websocket-client 0.56.0   
wheel            0.33.6  

Start at the exec_run entry point and trace how stdout and stderr settings affect the returned ExecResult. Add a regression test for both streams disabled, verifying that the exit_code is still reported while output remains empty.