Add notempty

[redixin]

Add notempty parameter. Pretty much the same as required but also check if value is empty string.

[sethvargo]

Duplicate of #44

[redixin]

Have read the thread. I agree that empty variables is a kind of misconfigurations or bugs in some scripts, and that's why we have required tag. We have this tag to detect misconfiguration and fail early.
Currently we use additional validation for such cases, but I guess we will just use another library to parse envs

[sethvargo]

Sounds great, thanks! Glad to hear you have a path forward 😄.