Empty mcp tool result error

[thoo]

Summary

Fixed MCPUtil.invoke_mcp_tool() to return "[]" instead of logging an error when MCP tools return empty content. Empty results (None, []) are valid outputs and should be treated consistently with
function tools. The None -> "[]" and [] -> "[]" behavior comes from the MCP Python SDK's FastMCP implementation.

Test plan

• Added comprehensive test coverage in test_mcp_fastmcp_behavior_verification() that verifies all FastMCP edge cases
• Enhanced FakeMCPServer to support custom content testing scenarios
• Ran full test suite - all tests pass
• Verified the fix handles None, [], {}, [{}], [[]] cases correctly

Issue number

Closes #1035

Checks

• I've added new tests (if relevant)
• I've added/updated the relevant documentation
• I've run make lint and make format
• I've made sure tests pass

[seratch]

This makes sense and looks good to me; thanks for sending this!