restJson error deserialization fails on empty body

[lucix-aws]

Describe the bug

Uncovered by #4740.

In scenarios where restjson error deserialization falls back to inspect the body, the SDK has always failed if the body is empty.

Before the above change, this code path would have only been hit when the error response did NOT contain the code as a header. Post-change, it is now hit when the headers are also missing the message.

We should consider this error message purely optional, the only value needed to deserialize to the appropriate type is the code.

Expected Behavior

deserialized error

Current Behavior

unmarshal error

Reproduction Steps

n/a

Possible Solution

No response

Additional Information/Context

No response

SDK version used

latest

Environment details (Version of Go (go version)? OS name and version, etc.)

n/a

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