Ensure config.path = None and config.path missing mean the same thing

[ianawilson]

Previously if path was present, it was expected to have a value. This allows path to be None, and for None and empty to act in the same way.

Originally I made this change because setting defaults to False and keyword to True didn't seem to be enough for me.

[davidmezzetti]

Just for my clarity, the PR supports adding a a config key of path: None? The current method assumes path wouldn't be there but this change takes it a step further and checks that if the key is there, it has a value?

[ianawilson]

Just for my clarity, the PR supports adding a a config key of path: None? The current method assumes path wouldn't be there but this change takes it a step further and checks that if the key is there, it has a value?

We talked a bit in community slack, but the gist is that I wanted None and missing to act the same. The title and description have been updated for clarity.