Update ActivityInputConverter for Activities with FunctionContext-only parameters

[nytian]

Fix issue #2741

When activity functions with only one parameter, the FunctionContext, would incorrectly have the FunctionContext set to null after ConvertAsync() at ActivityInputConverter. That's because the ContextConverter handles the source as null and TargetType as FunctionContext. This PR adds a specific case in the ActivityInputConverter to address this issue.

Pull request checklist

• My changes do not require documentation changes
  • Otherwise: Documentation PR is ready to merge and referenced in pending_docs.md
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• I have added all required tests (Unit tests, E2E tests)
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• My changes do not change the version of the WebJobs.Extensions.DurableTask package
  • Otherwise: major or minor version updates are reflected in /src/Worker.Extensions.DurableTask/AssemblyInfo.cs
• My changes do not add EventIds to our EventSource logs
  • Otherwise: Ensure the EventIds are within the supported range in our existing Windows infrastructure. You may validate this with a deployed app's telemetry. You may also extend the range by completing a PR such as this one.
• My changes should be added to v3.x branch.
  • Otherwise: This change only applies to Durable Functions v2.x and will not be merged to branch v3.x.

[nytian]

The E2E test I made:
Original error:

After changes in this PR