fix: handle empty Struct in valueToObject to prevent NullPointerException

[lizongbo]

When processing a protobuf Value containing an empty Struct, the valueToObject method would call structToMap which returns null for empty structs. This caused a NullPointerException when the result was used in stream collection operations.

The fix adds a null/empty check for STRUCT_VALUE case, returning an empty HashMap instead of delegating to structToMap when the struct has no fields.

Fixes #618

[gemini-code-assist]

Summary of Changes

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This pull request resolves a NullPointerException that arose when converting Protobuf Value objects to Java objects, specifically when an empty Struct was encountered. The change ensures that empty Struct values are correctly mapped to empty Java HashMap instances, thereby preventing runtime errors and improving the robustness of the data mapping logic.

Highlights

• NullPointerException Fix: Addresses a NullPointerException that occurred when the valueToObject method processed a Protobuf Value containing an empty Struct, as the structToMap method would return null in such cases.
• Empty Struct Handling: Implements a check within the valueToObject method for the STRUCT_VALUE kind. If the Struct is null or has no fields, an empty HashMap is now returned directly, preventing the NullPointerException.
• New Unit Tests: Adds A2ACommonFieldMapperTest.java with two new test cases (testValueToObject_WithEmptyStruct_ReturnsEmptyMap and testValueToObject_WithNestedEmptyStruct_ReturnsEmptyMap) to validate the correct handling of empty and nested empty structs.

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[gemini-code-assist]

Code Review

This pull request addresses a NullPointerException that occurs when processing an empty protobuf Struct. The fix introduces a check to handle empty structs within the valueToObject method, returning an empty HashMap to prevent the crash. The change is effective and is well-supported by new unit tests that cover both simple and nested empty struct scenarios. The suggested improvement to use Collections.emptyMap() for better performance and adherence to best practices remains valid.