Allow `envman add` to accept empty value

[Viktor Benei]

This would be super useful in case I want to create an environment list template file, which defines all the required environments but the values will be specified by someone else later, before use.

Maybe also add some option to the stored env item which defines whether the given env should be specified before it could be used with an envman run - similar to expand.

This way I could create a template envstore but I would mark all the required inputs, so that while those are empty/not filled in someone else can't run it with envman run.