Fix NPE in AsyncRetryableStage

[dagnir]

Motivation and Context

When retrying and checking for the 'x-amz-retry-after' or 'retry-after' header on the response, ensure that the response is not null first.

Modifications

Testing

Screenshots (if appropriate)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Checklist

• I have read the CONTRIBUTING document
• Local run of mvn install succeeds
• My code follows the code style of this project
• My change requires a change to the Javadoc documentation
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• My change is to implement 1.11 parity feature and I have updated LaunchChangelog

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[dagnir]

Correction on description/commit: this only relates to x-amz-retry-after since this is specific to the async retry stage, which never looked at retry-after. Furthermore, this code path is only hit if using retries 2.1 (off by default).