fix: correct fallback with no key no `.env` file

[tunakasif]

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Fixes the fall back behavior when no key is provided and no .env file is found. The previous implementation was raising an exception due to opening a file that does not exist.

Fixes: #27

[motdotla]

Thank you for this. I'll try to get to this soon, but I do have plans to ultimately merge dotenv-vault with dotenvx.

All my efforts are on dotenvx now - and then to follow merging dotenv-vault into dotenvx.