check for null headers in header callback

[sbiscigl]

Issue #, if available:

#3646

Description of changes:

When the S3Crt encounters a AWS_ERROR_HTTP_CHANNEL_THROUGHPUT_FAILURE error, we will segfault in the header callbacks because crt will pass use null headers. This checks for null headers and fails the callback if they are missing. This will allow the request to complete if the request has been configured to allow periods where no data is transferred by setting s3CrtConfig.lowSpeedLimit = 0;.

Check all that applies:

• Did a review by yourself.
• Added proper tests to cover this PR. (If tests are not applicable, explain.)
  Creating a AWS_ERROR_HTTP_CHANNEL_THROUGHPUT_FAILURE error is not easy at test time and would require specific setup. The fix was manually tested and verified using the code from the initial ticket.
• Checked if this PR is a breaking (APIs have been changed) change.
• Checked if this PR will not introduce cross-platform inconsistent behavior.
• Checked if this PR would require a ReadMe/Wiki update.

Check which platforms you have built SDK on to verify the correctness of this PR.

• Linux
• Windows
• Android
• MacOS
• IOS
• Other Platforms

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