Transcode tasks fail when source file type is not detected

[farski]

If a source file type cannot be determined by file-type, Artifact.Descriptor will equal {}.

Currently, the Transcode Task Fargate Execution state always expects that value to exist, and passes "$.Artifact.Descriptor.Extension" as a container override envar to the Fargate task run.

When the value doesn't exist, it results in a States.Runtime state machine error, which can't be caught, meaning the job fails but returns no error callbacks.