Make key and secret optional

[alexw23]

For use with IAM EC2/ECS/Task Roles

It looks like you had done this - however if statements don't work if the key is not set, a better method is to use array_key_exists

[patrickcarlohickman]

Thanks for taking the time to make a PR!

Would you mind changing those to !empty() checks?

The current code prevents creating the credentials if key or secret are false-y, but as you've seen, will throw an exception if they're not defined at all.

Your update to array_key_exists() will prevent the exception from occurring, but does not prevent the credentials from being set if key or secret are false-y.

A change to !empty() will protect against both scenarios.

Thanks again.

[patrickcarlohickman]

This functionality has been included in the latest release with the empty() checks. Thank you for your suggestion!