Change parseEnvKeys return type and warn on empty env keys

[abskptnyk]

fixes #14

summary:

• changed parseEnvKeys return type from map[string]struct{} to map[string]string
• added warning when key is present but value is empty

[vidya381]

loop returns on the first empty key so if multiple keys are empty only one gets reported. map iteration order is random so which one gets reported is non-deterministic. should collect all empty keys first then report them together.

[abskptnyk]

1. you're right about iterating over all keys
2. faced some issues while rebasing, causing me to force-push, which consequently closed this PR