Fix seg fault(s) in AuthV4Signer, test more combos with checksumming

[jeking3]

Issue #, if available:

V1183021468

Description of changes:

I observed a null pointer dereference in the following conditions:

1. Calling PutObject with no body (zero length object).
2. Setting the checksum algorithm to SHA256 (though likely any will do).
3. Providing a pre-calculated checksum (though likely any matching (2) will do).
4. Using payload signatures.

In multiple places, SDK code checks to see if the Content-Body is a nullptr before using it, but in AWSAuthV4Signer::SignRequest it was not doing so.

A second bug was observed when not using payload signatures, only when the scheme is HTTP. In this case the code assumed it was okay to call GetHeaderValue on Content-Length before checking if the header exists (it is not okay to do so). This was identified by the more rigorous testing.

Added a matrix test to verify more code paths in AWSAuthV4Signer and fixed the two seg faults in the area. The assumptions checked during testing should be scrutinized.

Tested this live against s3 and s3express endpoints using a workload that generates zero-length objects.

Check all that applies:

• Did a review by yourself: Yes
• Added proper tests to cover this PR. (If tests are not applicable, explain.): Yes
• Checked if this PR is a breaking (APIs have been changed) change: No
• Checked if this PR will not introduce cross-platform inconsistent behavior: No
• Checked if this PR would require a ReadMe/Wiki update: No

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