Now serializing empty lists

[tracyboehrer]

Fixes #279

[Copilot]

Pull Request Overview

This PR fixes issue #279 by updating the JSON serialization logic to handle empty lists differently based on the SKIP_EMPTY_LISTS compilation symbol. The changes include conditional test assertions for empty arrays, modifications in the serialization converter logic to skip empty collections when required, and updates in model initializers using the new C# 12 array literal syntax.

Reviewed Changes

Copilot reviewed 16 out of 16 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/tests/Microsoft.Agents.Model.Tests/StreamInfoTests.cs	Conditional assertions added to check different JSON outputs based on empty list serialization.
src/tests/Microsoft.Agents.Model.Tests/Resources/cps_typing_out.json	Expected JSON updated with empty arrays where applicable.
src/tests/Microsoft.Agents.Model.Tests/Resources/cps_suggestedactions_out.json	Expected JSON now includes empty arrays for various properties.
src/tests/Microsoft.Agents.Model.Tests/Resources/cps_greeting_out.json	Updated expected JSON with empty arrays for several fields.
src/tests/Microsoft.Agents.Model.Tests/Resources/cps_event_out.json	Expected JSON updated to reflect empty arrays in event payloads.
src/tests/Microsoft.Agents.Model.Tests/Resources/ComplexActivityPayload.json	Modified the expected value for "listenFor" from an empty array to null.
src/tests/Microsoft.Agents.Model.Tests/ObjectSerializationTests.cs	Wrapped tests with conditional compilation to assert serialization output for empty lists.
src/tests/Microsoft.Agents.Model.Tests/ConversationReferenceTests.cs	Updated expected JSON in conversation reference tests with empty array properties conditionally.
src/tests/Microsoft.Agents.Model.Tests/ConversationParametersTests.cs	Updated expected JSON output to include conditional serialization of empty collections.
src/tests/Microsoft.Agents.Extensions.Teams.Tests/Model/TestJson/MessagingExtensionActionResponse.json	Revised expected JSON output to account for empty arrays in nested properties.
src/tests/Microsoft.Agents.Extensions.Teams.Tests/Model/TestJson/MessagingExtensionAction.json	Nested activity preview properties now include empty arrays where applicable.
src/libraries/Extensions/Microsoft.Agents.Extensions.Teams/Serialization/Converters/MessagingExtensionAttachmentConverter.cs	Added logic to correctly read and write extension data using the new JSON APIs.
src/libraries/Core/Microsoft.Agents.Core/Serialization/Converters/ConnectorConverter.cs	Introduced conditional checks to skip empty list properties during serialization and adjusted reading logic.
src/libraries/Core/Microsoft.Agents.Core/Models/VideoCard.cs	Updated constructor to initialize collection properties using the new array literal syntax.
src/libraries/Core/Microsoft.Agents.Core/Models/BasicCard.cs	Updated initializer to coalesce nulls with new array literal syntax for Images and Buttons.
src/libraries/Core/Microsoft.Agents.Core/Models/Activity.cs	Updated constructors to initialize collection properties with empty arrays using the new literal syntax.

Comments suppressed due to low confidence (1)

src/libraries/Core/Microsoft.Agents.Core/Serialization/Converters/ConnectorConverter.cs:69

• Confirm that skipping serialization of empty lists when SKIP_EMPTY_LISTS is defined does not break external contracts expecting these fields. If empty arrays are required by consumers of the JSON, consider revisiting this logic.

if (propertyValue is IList list)