Handle null when serializing enum to string

[RanVaknin]

Motivation and Context

#3861

Modifications

Updated MemberCopierSpec.java to generate a null check before calling `entry.toString() when serializing enums to strings.

Testing

• Updated codegen fixture files with the expected code generated content
• Added a unit test to verify that null enum doesn't throw NPE.

[RanVaknin]

Flakey integ test failure S3TransferManagerDownloadPauseResumeIntegrationTest , should be safe to merge.

[zoewangg]

Can we add a changelog entry? Changelog entry is not required only for house keeping changes such as test code or README update

[joviegas]

Can we add a test case in the protocol test , may be src/main/resources/software/amazon/awssdk/protocol/suites/cases/json-core-input.json ?
something like

{
  "description": "List of enums with null values are marshalled correctly without NPE",
  "given": {
    "input": {
      "ListOfEnums": [
        "EnumValue1",
        null,
        "EnumValue2"
      ]
    }
  },
  "when": {
    "action": "marshall",
    "operation": "AllTypes"
  },
  "then": {
    "serializedAs": {
      "body": {
        "jsonEquals": "{\"ListOfEnums\": [\"EnumValue1\", null, \"EnumValue2\"]}"
      }
    }
  }
}

same for maps so we know what we are sending on the wire

[sonarqubecloud]

Quality Gate passed

Issues
4 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

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