Linux: Empty environment variables should use defaults

[clbarnes]

I believe appdirs does not conform to the XDG spec when environment variables exist, but are empty.

The XDG spec says:

If $XDG_DATA_HOME is either not set or empty, a default equal to $HOME/.local/share should be used.

However, the implementation here uses path = os.getenv('XDG_DATA_HOME', os.path.expanduser("~/.local/share")), which is correct when the environment variable does not exist, but incorrect when it exists but is empty.

I propose a helper function like

def getenv(name, default=None):
    value = os.getenv(name)
    if not value:
        value = default
    return value

and using that wherever os.getenv is currently used.

This is a breaking change, but necessary for conformance with the XDG spec.