`_` added to resolved env vars if an empty string is used as `envVarsPrefix`

[saledjenic]

If envVarsPrefix is set to an empty string env var being used starts with _. In that case, if I want to set MY_VAR variable, I have to set it as _MY_VAR to be recognized.

[nitely]

I think envVarsPrefix should always be provided, otherwise unintended envars may end up being loaded. If the passed envVarsPrefix is empty, envvars should be ignored (do not use them to populate defaults).