cli: Fix swh-scheduler command

Add a simple fix for currently broken swh-scheduler command (see stacktrace below).

The command should be removed soon but while it is still here, it should work.

Executing test script test_01_loader_git.sh
Scheduling the loading of the git repository located at https://forge.softwareheritage.org/source/swh-loader-tar.git[test_01_loader_git.sh] 

Traceback (most recent call last):
  File "/srv/softwareheritage/venv/bin/swh-scheduler", line 10, in <module>
    sys.exit(main())
  File "/srv/softwareheritage/venv/lib/python3.6/site-packages/swh/scheduler/cli/__init__.py", line 72, in main
    return cli(auto_envvar_prefix='SWH_SCHEDULER')
  File "/srv/softwareheritage/venv/lib/python3.6/site-packages/click/core.py", line 764, in __call__
    return self.main(*args, **kwargs)
  File "/srv/softwareheritage/venv/lib/python3.6/site-packages/click/core.py", line 717, in main
    rv = self.invoke(ctx)
  File "/srv/softwareheritage/venv/lib/python3.6/site-packages/click/core.py", line 1134, in invoke
    Command.invoke(self, ctx)
  File "/srv/softwareheritage/venv/lib/python3.6/site-packages/click/core.py", line 956, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/srv/softwareheritage/venv/lib/python3.6/site-packages/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
  File "/srv/softwareheritage/venv/lib/python3.6/site-packages/click/decorators.py", line 17, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "/srv/softwareheritage/venv/lib/python3.6/site-packages/swh/scheduler/cli/__init__.py", line 36, in cli
    loglevel=ctx.obj['log_level'], colorize=False,
TypeError: 'NoneType' object is not subscriptable
An error occurred when running test script test_01_loader_git.sh

Depends on swh-core!59 (closed)

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Migrated from D1464 (view on Phabricator)