fix(aws,bedrock): don't require Content-Type for body-less SigV4 requests (#1229)

The AWS SigV4 signing path threw AnthropicInvalidDataException for any
request without a body and without an explicit Content-Type header
(e.g., GET, DELETE, POST archive). This made the AwsBackend unusable
for read/list/delete/archive operations out of the box.

SigV4 does not require a Content-Type header for body-less requests.
Only throw when a body is present without a content type.

Found during the CMA-on-AWS M2 bug bash.