Fix handling of --dump-config when there is no existing config file and when the existing config file is empty

[mtkennerly]

I ran into this problem when trying out the tool on a brand new install:

$ cmake-format --dump-config yaml
Traceback (most recent call last):
  File "c:\python\3.7-32\lib\runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "c:\python\3.7-32\lib\runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "C:\Python\3.7-32\Scripts\cmake-format.exe\__main__.py", line 9, in <module>
  File "c:\python\3.7-32\lib\site-packages\cmake_format\__main__.py", line 389, in main
    dump_config(args, config_dict, sys.stdout)
  File "c:\python\3.7-32\lib\site-packages\cmake_format\__main__.py", line 253, in dump_config
    cfg = configuration.Configuration(**config_dict)
TypeError: type object argument after ** must be a mapping, not NoneType

With the fix:

$ cmake-format --dump-config yaml
line_width: 80
tab_size: 2
max_subargs_per_line: 3
separate_ctrl_name_with_space: false
separate_fn_name_with_space: false
dangle_parens: false
[...]

[cheshirekow]

I've tried to reproduce this error in a number of environments and I'm unable to do so. I also can't see how config_dict can ever be None in this location. Are you able to reproduce this error reliably?

[cheshirekow]

Hm... ok so I am only able to reproduce the problem with an empty configuration file. It only appears to be a problem with yaml configurations. Presumably apparently yaml.load returns None instead of an empty dictionary if a yaml file is empty... which is annoying.

Can you confirm that this error is not present if there is no config file in play? You said you had a brand new install but it seems like you had created a config file.

[cheshirekow]

I'm unable to reproduce with missing config but I fixed the case where yaml.load returns None. If you can reproduce the no-config case please let me know.