Test code path for when the KMS client supplier returns NULL

[bdonlan]

This branch is not taken in the test suite, and should be tested:

https://github.com/awslabs/aws-encryption-sdk-c/blob/5b77a0a9b7131ffd3f32b6057cf42bfeaf0315b3/aws-encryption-sdk-cpp/source/kms_keyring.cpp#L100-L103