Empty config file leads to confusing error message

[micahellison]

Bug Report

Environment

• jrnl --diagnostic output:

jrnl: v2.8.4-beta2
Python: 3.8.2 (tags/v3.8.2:7b3ab59, Feb 25 2020, 23:03:10) [MSC v.1916 64 bit (AMD64)]
OS: Windows 10

• Install method: pipx

Current Behavior

If your config file is empty, you get a stack trace and a TypeError: 'NoneType' object is not iterable message.

Expected Behavior

It would be nice to have an error message that explains that the config file can't be parsed. Maybe show the path of the config file too.

Repro Steps

• Empty the contents of your jrnl.yaml config file and save it
• Run jrnl

Other Information

Stack trace:

Traceback (most recent call last):
  File "c:\users\micah\.pyenv\pyenv-win\versions\3.8.2\lib\runpy.py", line 193, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "c:\users\micah\.pyenv\pyenv-win\versions\3.8.2\lib\runpy.py", line 86, in _run_code
    exec(code, run_globals)
  File "C:\Users\micah\.local\bin\jrnl.exe\__main__.py", line 7, in <module>
  File "C:\Users\micah\.local\pipx\venvs\jrnl\lib\site-packages\jrnl\cli.py", line 34, in cli
    return run(args)
  File "C:\Users\micah\.local\pipx\venvs\jrnl\lib\site-packages\jrnl\jrnl.py", line 39, in run
    config = install.load_or_install_jrnl(args.config_file_path)
  File "C:\Users\micah\.local\pipx\venvs\jrnl\lib\site-packages\jrnl\install.py", line 92, in load_or_install_jrnl
    upgrade_config(config, alt_config_path)
  File "C:\Users\micah\.local\pipx\venvs\jrnl\lib\site-packages\jrnl\install.py", line 28, in upgrade_config
    missing_keys = set(default_config).difference(config_data)
TypeError: 'NoneType' object is not iterable

A resolution to this issue should have:

• An easy-to-understand error message when the config file can't be parsed
• A BDD test that confirms the error message appears with an empty config file