Use empty strings for credentials if not defined in env vars

[nickday]

Thanks for making this! I'm using it with Amplify in a Lambda resolver. It works fine in the cloud, but when mocking locally using amplify mock api the AWS credentials are not set and so it attempts to create credentials using undefined producing:

TypeError [ERR_INVALID_ARG_TYPE]: The "key" argument must be of type string or an instance of Buffer, TypedArray, DataView, or KeyObject. Received undefined

This change makes it fallback to empty strings instead, and mocking in Amplify now works 🎉

[amcdnl]

published