fix: coalesce None env values to empty string

[StykMartin]

2026-02-01 16:55:12,958 bkr.labcontroller.provision DEBUG Launching power script /etc/beaker/power-scripts/docker (attempt 1) with env {'LC_FQDN': 'lc-fedora41', 'HOSTNAME': 'lc-fedora41', 'DISTTAG': 'f41container', 'PWD': '/beaker', 'FBR': 'f41', 'HOME': '/root', 'FGC': 'f41', 'REGISTER_MACHINES': '1', 'PYTHONPATH': '/beaker/Common:/beaker/LabController/src:/beaker/Client/src', 'TERM': 'xterm', 'SHLVL': '0', 'PATH': '/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin', '_': '/usr/bin/python3', 'LC_CTYPE': 'C.UTF-8', 'power_address': '/var/run/docker.sock', 'power_id': 'beaker-machine-1', 'power_user': None, 'power_pass': None, 'power_mode': 'interrupt'}
2026-02-01 16:55:12,972 bkr.labcontroller.provision ERROR Error processing command 1
Traceback (most recent call last):
  File "/beaker/LabController/src/bkr/labcontroller/provision.py", line 161, in handle
    handle_power(self.conf, command)
    ~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^
  File "/beaker/LabController/src/bkr/labcontroller/provision.py", line 275, in handle_power
    p = MonitoredSubprocess([script], env=env,
            stdout=subprocess.PIPE, stderr=subprocess.STDOUT,
            timeout=300)
  File "/beaker/LabController/src/bkr/labcontroller/concurrency.py", line 104, in __init__
    super(MonitoredSubprocess, self).__init__(*args, **kwargs)
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^
  File "/usr/lib64/python3.13/site-packages/gevent/subprocess.py", line 819, in __init__
    self._execute_child(args, executable, preexec_fn, close_fds,
    ~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
                        pass_fds, cwd, env, universal_newlines,
                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    ...<5 lines>...
                        gid, gids, uid, umask,
                        ^^^^^^^^^^^^^^^^^^^^^^
                        start_new_session, process_group)
                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib64/python3.13/site-packages/gevent/subprocess.py", line 1849, in _execute_child
    raise child_exception
TypeError: expected str, bytes or os.PathLike object, not NoneType