Config doesn't read the defaut.yml file if custom-environment-variables.yml (or anyother .yml file) is empty about python-config2

[ShethR]

https://github.com/grimen/python-config2/blob/master/config2/config.py#L246

if say custom-environment-variables.yml is empty,
env_variables_file.data will be either None., or give KeyError. Performing copy() on a None object throws error
** AttributeError: 'NoneType' object has no attribute 'copy'**

Once the error is raised,
These lines are never executed.
https://github.com/grimen/python-config2/blob/master/config2/config.py#L248-L258

[grimen]

Closed by 470f573 - v0.2.1