handle NOT_SET as option for enumvalue

[jmklix]

Issue #, if available: Add code gen changes from this PR: https://github.com/aws/aws-sdk-cpp/pull/2695

Description of changes:

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• [X] Did a review by yourself.
• [ ] Added proper tests to cover this PR. (If tests are not applicable, explain.)
• [X] Checked if this PR is a breaking (APIs have been changed) change.
• [X] Checked if this PR will not introduce cross-platform inconsistent behavior.
• [X] Checked if this PR would require a ReadMe/Wiki update.

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