Handle falsey return values from sources about batconf HOT 1 OPEN

Proposed solution:
We will only officially support String values in configuration sources. Converting string values to other types will be the responsibility of the application.

This is more aligned with Environment Variable, and CLI sources, and seems to be common among other configuration storage services.

We will not actively prevent custom sources from returning non-string values, but Falsey values will continue to be a known problem if you choose to do so.

We may consider a more complex solution, raising exceptions instead of returning None, in the future if a compelling use-case presents its self.

from batconf.