Check rmw identifier

[mikaelarguedas]

Potential fix for #120

• Linux
• Linux-aarch64
• macOS
• Windows

without the patch:

Traceback (most recent call last):
  File "/opt/ros/bouncy/bin/ros2", line 11, in <module>
    load_entry_point('ros2cli==0.5.2', 'console_scripts', 'ros2')()
  File "/opt/ros/bouncy/lib/python3.6/site-packages/ros2cli/cli.py", line 69, in main
    rc = extension.main(parser=parser, args=args)
  File "/opt/ros/bouncy/lib/python3.6/site-packages/ros2node/command/node.py", line 39, in main
    return extension.main(args=args)
  File "/opt/ros/bouncy/lib/python3.6/site-packages/ros2node/verb/list.py", line 34, in main
    with NodeStrategy(args) as node:
  File "/opt/ros/bouncy/lib/python3.6/site-packages/ros2cli/node/strategy.py", line 29, in __init__
    spawn_daemon(args)
  File "/opt/ros/bouncy/lib/python3.6/site-packages/ros2cli/node/daemon.py", line 72, in spawn_daemon
    stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL, **kwargs)
  File "/usr/lib/python3.6/subprocess.py", line 709, in __init__
    restore_signals, start_new_session)
  File "/usr/lib/python3.6/subprocess.py", line 1275, in _execute_child
    restore_signals, start_new_session, preexec_fn)
TypeError: expected str, bytes or os.PathLike object, not NoneType

with the patch:

# ros2 node list
Unable to get rmw_implementation_identifier, try specifying the implementation to use via the 'RMW_IMPLEMENTATION' environment variable