swh-fuse: fails with "'TypeError: Cannot merge a <class 'dict'> with a <class 'NoneType'>" when conffile is empty or commented out

When ~/.config/swh/global.yml is either empty or entirely commented out (with # lines), swh fuse mount fails with the following type error:

Traceback (most recent call last):
  File "/home/zack/.virtualenvs/swh/bin/swh", line 33, in <module>
    sys.exit(load_entry_point('swh.core', 'console_scripts', 'swh')())
  File "/home/zack/dati/projects/sw-heritage/git/swh-environment/swh-core/swh/core/cli/__init__.py", line 135, in main
    return swh(auto_envvar_prefix="SWH")
  File "/home/zack/.virtualenvs/swh/lib/python3.8/site-packages/click/core.py", line 829, in __call__
    return self.main(*args, **kwargs)
  File "/home/zack/.virtualenvs/swh/lib/python3.8/site-packages/click/core.py", line 782, in main
    rv = self.invoke(ctx)
  File "/home/zack/.virtualenvs/swh/lib/python3.8/site-packages/click/core.py", line 1259, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/home/zack/.virtualenvs/swh/lib/python3.8/site-packages/click/core.py", line 1256, in invoke
    Command.invoke(self, ctx)
  File "/home/zack/.virtualenvs/swh/lib/python3.8/site-packages/click/core.py", line 1066, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/home/zack/.virtualenvs/swh/lib/python3.8/site-packages/click/core.py", line 610, in invoke
    return callback(*args, **kwargs)
  File "/home/zack/.virtualenvs/swh/lib/python3.8/site-packages/click/decorators.py", line 21, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "/home/zack/dati/projects/sw-heritage/git/swh-environment/swh-fuse/swh/fuse/cli.py", line 67, in fuse
    conf = config.merge_configs(DEFAULT_CONFIG, conf)
  File "/home/zack/dati/projects/sw-heritage/git/swh-environment/swh-core/swh/core/config.py", line 224, in merge_configs
    raise TypeError("Cannot merge a %s with a %s" % (type(base), type(other)))
TypeError: Cannot merge a <class 'dict'> with a <class 'NoneType'>

that is unfortunate, given one of the main interests of YAML for configuration files is the ability to comment out stuff (and an empty configuration file is an entirely legitimate thing as well).

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Migrated from T2710 (view on Phabricator)