Standardize the way SdkBytes are unmarshalled when they're part of a payload and the service returns no content.

[millems]

After this change, when SdkBytes are modeled as a payload, the SDK will always return a non-null empty SdkBytes. When SdkBytes are modeled as a field, they will be null if the service did not specify them.

This issue was brought to our attention when #2982 standardized the Netty HTTP client's behavior. Previously, chunked encoded responses would behave differently than non-chunked encoded payloads. While that change made the behavior consistent, it had the unintentional effect of the SDK now returning null for blob-returning operations (like Lambda's invoke) instead of an empty SdkBytes (unless the Lambda response used chunked encoding, in which case the result would have been null even under the old behavior).

This change standardizes this behavior across all protocols. The table at the end of this PR summarizes the current and new behavior.

Other changes in this PR:

1. Upgrade wiremock to wiremock-jre8. The old package is deprecated, and the newest package was required for junit 5 extension support. (Note: Wiremock could not be upgraded for the HTTP client tests, because of a bug in the latest wiremock that caused wiremock to not detect HEAD requests properly)

Client Type	SDK Protocol	SdkBytes Location	Response Encoding	Previous Value	New Value
SYNC	JSON	PAYLOAD	CONTENT_LENGTH	empty	empty
SYNC	JSON	PAYLOAD	CHUNKED	empty	empty
SYNC	JSON	FIELD	CONTENT_LENGTH	null	null
SYNC	JSON	FIELD	CHUNKED	null	null
SYNC	XML	PAYLOAD	CONTENT_LENGTH	empty	empty
SYNC	XML	PAYLOAD	CHUNKED	empty	empty
SYNC	XML	FIELD	CONTENT_LENGTH	null	null
SYNC	XML	FIELD	CHUNKED	null	null
SYNC	QUERY	PAYLOAD	CONTENT_LENGTH	exception	empty
SYNC	QUERY	PAYLOAD	CHUNKED	exception	empty
SYNC	QUERY	FIELD	CONTENT_LENGTH	exception	null
SYNC	QUERY	FIELD	CHUNKED	exception	null
ASYNC	JSON	PAYLOAD	CONTENT_LENGTH	null*	empty
ASYNC	JSON	PAYLOAD	CHUNKED	null	empty
ASYNC	JSON	FIELD	CONTENT_LENGTH	null	null
ASYNC	JSON	FIELD	CHUNKED	null	null
ASYNC	XML	PAYLOAD	CONTENT_LENGTH	null	empty
ASYNC	XML	PAYLOAD	CHUNKED	null	empty
ASYNC	XML	FIELD	CONTENT_LENGTH	null	null
ASYNC	XML	FIELD	CHUNKED	null	null
ASYNC	QUERY	PAYLOAD	CONTENT_LENGTH	null	empty
ASYNC	QUERY	PAYLOAD	CHUNKED	null	empty
ASYNC	QUERY	FIELD	CONTENT_LENGTH	null	null
ASYNC	QUERY	FIELD	CHUNKED	null	null

* This was the behavior that was changed with #2982. Prior to that change, it would have been 'empty'.

[cenedhryn]

nice utility

[sonarqubecloud]

SonarCloud Quality Gate failed.   

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