Python: keep empty tool results through the prompt round trip (#14686)

### Motivation and Context

Fixes #14685.

Since #14472, `ChatHistory.add_tool_message("")` keeps an empty tool
result. But that result doesn't survive the XML prompt round trip that
`KernelFunctionFromPrompt` performs on every chat completion call.
`to_element()` renders it as a self-closing `<function_result
id="call_1" />`. When `from_element()` parses that back, it reads
`element.text`, which is `None` for an empty element.
`ChatMessageContent` then stringifies the value, so the model receives
the literal tool result `"None"` instead of an empty one.

### Description

- `FunctionResultContent.from_element` now maps a missing element text
to `""`. Non-empty results are unchanged.
- Added `test_element_round_trip_keeps_result` (empty and non-empty),
which serialises the element to XML and parses it back.
- Added `test_chat_history_prompt_round_trip_keeps_empty_tool_result`,
which reproduces the issue end to end through `to_prompt()` /
`from_rendered_prompt()`.

Before (main `4bb64ee`, macOS arm64, Python 3.12):

```
{'role': 'tool', 'content': 'None', 'tool_call_id': 'call_1'}
```

After:

```
{'role': 'tool', 'content': '', 'tool_call_id': 'call_1'}
```

Tests, run from `python/`:

- `pytest tests/unit/contents`: 421 passed on `main`, 424 passed with
this change.
- With the source change reverted, the two empty-result tests fail: `2
failed, 25 passed` in `test_function_result_content.py`.
- `ruff check` / `ruff format --check` on both files are clean.

This doesn't overlap the open #14375 (the
`from_function_call_content_and_result` factory) or #14480 (the
`add_tool_message` path), and it doesn't touch the JSON `serialize()` /
`restore_chat_history()` path, which already keeps `""`.

### Contribution Checklist

- [x] The code builds clean without any errors or warnings
- [x] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [x] All unit tests pass, and I have added new tests where possible
- [x] I didn't break anyone 😄

AI-assisted: yes ([Cana](https://cana.build))

Author: iamthehimansh

python/semantic_kernel/contents/function_result_content.py

@@ -116,7 +116,9 @@ def from_element(cls: type[_T], element: Element) -> _T:
         """Create an instance from an Element."""
         if element.tag != cls.tag:
             raise ContentInitializationError(f"Element tag is not {cls.tag}")  # pragma: no cover
-        return cls(id=element.get("id", ""), result=element.text, name=element.get("name", None))
+        # An empty result renders as a self-closing element, whose text parses back as None.
+        result = element.text if element.text is not None else ""
+        return cls(id=element.get("id", ""), result=result, name=element.get("name", None))
 
     @classmethod
     def from_function_call_content_and_result(

python/tests/unit/contents/test_function_result_content.py

@@ -2,10 +2,12 @@
 
 
 from unittest.mock import Mock
+from xml.etree.ElementTree import XML, tostring  # nosec
 
 import pytest
 from pydantic import BaseModel, ConfigDict, Field
 
+from semantic_kernel.contents.chat_history import ChatHistory
 from semantic_kernel.contents.chat_message_content import ChatMessageContent
 from semantic_kernel.contents.function_call_content import FunctionCallContent
 from semantic_kernel.contents.function_result_content import FunctionResultContent
@@ -123,6 +125,21 @@ def test_to_cmc():
     assert cmc.items[0].result == "test-result"
 
 
+@pytest.mark.parametrize("result", ["", "test-result"], ids=["empty", "non_empty"])
+def test_element_round_trip_keeps_result(result: str):
+    frc = FunctionResultContent(id="call_1", name="my-plugin-do", result=result)
+    restored = FunctionResultContent.from_element(XML(tostring(frc.to_element(), encoding="unicode")))
+    assert restored.result == result
+    assert restored.to_dict() == {"tool_call_id": "call_1", "content": result}
+
+
+def test_chat_history_prompt_round_trip_keeps_empty_tool_result():
+    history = ChatHistory()
+    history.add_tool_message("", tool_call_id="call_1", function_name="my-plugin-do")
+    parsed = ChatHistory.from_rendered_prompt(history.to_prompt())
+    assert parsed.messages[0].to_dict() == {"role": "tool", "content": "", "tool_call_id": "call_1"}
+
+
 def test_serialize():
     class CustomResultClass:
         def __init__(self, result):