Handle empty backup test requests

[cannontrodder]

The AWS reference implementation now handles the empty test request correctly when the probe header uses different casing.

Verified with upload/probe tests, API tests, build and formatting checks.

@LiamMorrow I see how you split the backends up so they can support particular features. What I might do is take a look at an AWS-hosted backend as well and see if there's a really cheap way to do it. If you've got that Docker container there, it shouldn't be too difficult, but those are famous last words.

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[LiamMorrow]

Thanks for this @cannontrodder I don't have an AWS account to test these changes so having you here fixing it up makes my day :)

@LiamMorrow I see how you split the backends up so they can support particular features. What I might do is take a look at an AWS-hosted backend as well and see if there's a really cheap way to do it. If you've got that Docker container there, it shouldn't be too difficult, but those are famous last words.

Famous last words indeed. I'd love to make it simpler to run, but even right now it is not that friendly. I'd definitely be open to something like a "lambda mode" for the dotnet app which just handles a lambda request for backup and quits, but it's not in a great state ATM to have that setup