Rule lambdaNotEncryptedWithKms should not check for KMS when env vars are not being used

[msnook]

• terrascan version: Latest Docker image

Description

Some of my lambda functions do not use environment variables, so there is no need to define a kms_key_arn for the function to use. However, it appears as though Terrascan is always looking for kms_key_arn and fails the rule even if env vars are not being used. This forces me to have to skip the rule for that lambda function. It would be nice if this rule failed during the Terrascan only if kms_key_arn is absent and env vars are used.