Logging the literal string "None" is silently dropped (payload becomes empty)

- [x] I determined this is the correct repository in which to report this bug.

## Summary of the issue

**Context**
Using `CloudLoggingHandler` / `StructuredLogHandler` via the Python standard logging module, I logged the string `"None"` (e.g. `logging.getLogger().info("None")`).

**Expected Behavior:**
The log entry payload should contain the text `"None"`, just like any other string.

**Actual Behavior:**
The message is silently dropped: the parsed payload becomes `None` (empty), so the log content disappears.

## API client name and version

google-cloud-logging v3.15.0 (also present on current `main`)

## Reproduction steps: code

```python
import logging
from google.cloud.logging_v2.handlers import handlers as H

class FakeHandler(logging.Handler):
pass

h = FakeHandler()
h.setFormatter(logging.Formatter())

def rec(msg):
return logging.LogRecord("n", logging.INFO, "p", 1, msg, (), None)

for msg in ["None", "hello", ""]:
print(repr(msg), "->", repr(H._format_and_parse_message(rec(msg), h)))
```

## Reproduction steps: actual results

```
'None' -> None # the user's message is dropped
'hello' -> 'hello'
'' -> ''
```

## Reproduction steps: expected results

```
'None' -> 'None' # the literal string should be preserved
'hello' -> 'hello'
'' -> ''
```

## OS & version + platform

Any (logic bug, platform-independent)

## Python environment

Python 3.12

## Additional context

Root cause in `google/cloud/logging_v2/handlers/handlers.py` (`_format_and_parse_message`):

```python
if message != "None":
passed_json_fields["message"] = message
...
return message if message != "None" else None
```

The intent appears to be detecting a record whose `msg` is the Python object `None` (which `logging.Formatter` renders as the string `"None"`). But comparing the *formatted string* against `"None"` also matches a legitimate user message of the literal text `"None"`, dropping it. Detecting emptiness from `record.msg is None` (before formatting) rather than from the formatted output would avoid the false positive.

Commencez dans google/cloud/logging_v2/handlers/handlers.py, au niveau de _format_and_parse_message, puis exécutez la reproduction fournie de LogRecord pour "None", "hello" et "". C'est terminé lorsque la chaîne littérale "None" reste dans la charge utile analysée, tandis que les cas existants conservent leurs résultats attendus.

Rédigé par le modèle d'indexation à partir du texte de l'issue.