In the output parameters of the tool, properties annotated with @McpToolParam cannot be null

For tools annotated with @McpTool, if the output is a custom class, then any properties within that class annotated with @McpToolParam must not be null, otherwise errors will occur：Validation failed: structuredContent does not match tool outputSchema. Validation errors: [$.current: 已找到 null，必须是 object]

How can I return a null value? Thank you.

Environment
mcp-annotations: 0.6.0
spring-ai: 1.1.0-M4
protocol: stateless

Minimal Complete Reproducible example

@Service
public class TestService {
    @McpTool
    public TestResponse test() {
        return new TestResponse();
    }

    public static class TestResponse {
        @McpToolParam(description = "test", required = false)
        private String text;

        public String getText() {
            return text;
        }

        public void setText(String text) {
            this.text = text;
        }
    }
}

Start with the @McpTool and @McpToolParam processing shown by the TestService and TestResponse reproduction, then reproduce the structuredContent validation error with the optional text property left null. Done means a custom tool output with an @McpToolParam(required = false) null property is accepted without the reported schema-validation failure.