Doubleclick in Table and Tree with data==None triggers IndexError

### Describe the bug

doubleclick in table with data==None triggers IndexError

example below results in error below

this might be by design, but since None is an acceptable value for data per the documentation, and it makes perfect sense to have an empty table in an application (mine is empty until the user triggers a db query), this behavior is unexpected to me

not a disaster - why would a user click there anyway If there is no data - but also not great since it crashes the application

```
Traceback (most recent call last):
File ".../.venv/lib/python3.14/site-packages/rubicon/objc/api.py", line 412, in __call__
result = self.py_method(py_self, *args)
File ".../.venv/lib/python3.14/site-packages/toga_cocoa/widgets/table.py", line 127, in onDoubleClick_
clicked = self.interface.data[self.clickedRow]
File ".../.venv/lib/python3.14/site-packages/toga/sources/list_source.py", line 135, in __getitem__
return self._data[index]
IndexError: list index out of range
```

### Steps to reproduce
```
import toga

class MyApp(toga.App):
def startup(self):
self.main_window = toga.MainWindow()
self.main_window.content = toga.Table(
headings=["Name", "Age"],
# data=[
# ("Arthur Dent", 42),
# ("Ford Prefect", 37),
# ("Tricia McMillan", 38),
# ],
data=None,
multiple_select=False,
)
self.main_window.show()

if __name__ == '__main__':
app = MyApp("Realistic App", "org.beeware.realistic")
app.main_loop()
```

### Expected behavior

no exception

### Screenshots

_No response_

### Environment

macOS 24.6.0
Python 3.14.0 (main, Oct 7 2025, 09:34:52) [Clang 17.0.0 (clang-1700.0.13.3)] on darwin
Toga: toga-0.5.2

### Logs

```

```

### Additional context

_No response_

Start with toga_cocoa/widgets/table.py at onDoubleClick_ and toga/sources/list_source.py at __getitem__, then run the supplied empty-data reproduction. Check the corresponding table and tree double-click behavior; done means clicking an empty widget raises no IndexError.

Written by the indexing model from the issue text.