Need clarity (perhaps in documentation) regarding when a value returned from DotEnv.dict() can be None

[posita]

The return type of DotEnv.dict() is Dict[str, Optional[str]], but it's not clear under what circumstances a value would be None. I haven't delved into the parsing code, but it's not clear to me, even after looking at (e.g.) resolve_variables, parse, etc.

Under what circumstances would one expect to receive a value of None for a particular key? Can this be documented?

[bbc2]

This possibility was added in #220. It makes it possible to have a variable name with no value in the .env file (i.e. just <name> instead of <name>=<value>), which is reflected as the name being associated with None in the resulting dict.

I agree this should be documented, at least for dotenv_values (DotEnv.dict is a bit less important because it's an internal API).