Fix the uniqueItems implementation to accomodate undefined values nested inside lists of objects

[pose]

Description of changes: I'm explicitly adding handling and test cases for scenarios where undefined and null values are passed.

I also added a new validation test for restJson1:

  ● RestJsonMalformedUniqueItemsStructureMissingKeyList:MalformedRequest

    expect(received).toBe(expected) // Object.is equality

    Expected: 400
    Received: 500

      4818 |
      4819 |   expect(testFunction.mock.calls.length).toBe(0);
    > 4820 |   expect(r.statusCode).toBe(400);
           |                        ^
      4821 |   expect(r.headers["x-amzn-errortype"]).toBeDefined();
      4822 |   expect(r.headers["x-amzn-errortype"]).toBe("ValidationException");
      4823 |

      at Object.<anonymous> (test/functional/restjson1.spec.ts:4820:24)

Locally, after yarn linking the new changes, the error is gone which indicated that the regression has been fixed.

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