Mengla/fix-output-null

[melionel]

Description

fix agentframework adapter to make sure the output in response complete event not null.

otherwise, it will cause agents service save response failed with exception:
System.ArgumentNullException: Value cannot be null. (Parameter 'source')
at System.Linq.ThrowHelper.ThrowArgumentNullException(ExceptionArgument argument)
at System.Linq.Enumerable.Select[TSource,TResult](IEnumerable1 source, Func2 selector)
at Azure.AI.Agents.Services.Storage.V2.Models.Extensions.ToStorageModel(Response apiModel, String projectId, IEnumerable1 inputItemRefs, Dictionary2 extensionData, IReadOnlyDictionary2 additionalMetadata) in /mnt/vss/_work/1/s/src/azureml-api/src/Agents/Services/Storage/V2/Models/Extensions.cs:line 62 at Azure.AI.Agents.Services.ContainerAgents.Core.Managers.ContainerResponseManager.SaveResponseAsync(Response response, AgentId agentId, List1 inputItems, ProjectConnectionString connectionString, String conversationId, CancellationToken cancellationToken, Boolean existing, IReadOnlyDictionary`2 telemetryMetadata) in /mnt/vss/_work/1/s/src/azureml-api/src/Agents/Services/ContainerAgents/Core/Managers/ContainerResponseManager.cs:line 154

All SDK Contribution checklist:

• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.

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• Pull request includes test coverage for the included changes.

[github-actions]

Thank you for your contribution @melionel! We will review the pull request and get back to you soon.