aider: Added a test for AutoCompleter that passes in a non-existent file and checks that self.words is an empty set.

# Context:
USER: add a test for `AutoCompleter` that passes in a non-existant file. make sure self.words is empty.
ASSISTANT: To add a test for `AutoCompleter` that passes in a non-existent file, we need to modify the `tests/test_io.py` file. We will add a new test method `test_autocompleter_with_non_existent_file` in the `TestInputOutput` class.

Here is the change:

```python
tests/test_io.py
<<<<<<< ORIGINAL
import os
import unittest
from unittest.mock import patch

from aider.io import InputOutput

class TestInputOutput(unittest.TestCase):
    def test_no_color_environment_variable(self):
        with patch.dict(os.environ, {"NO_COLOR": "1"}):
            io = InputOutput()
            self.assertFalse(io.pretty)

if __name__ == "__main__":
    unittest.main()
=======
import os
import unittest
from unittest.mock import patch

from aider.io import InputOutput, AutoCompleter

class TestInputOutput(unittest.TestCase):
    def test_no_color_environment_variable(self):
        with patch.dict(os.environ, {"NO_COLOR": "1"}):
            io = InputOutput()
            self.assertFalse(io.pretty)

    def test_autocompleter_with_non_existent_file(self):
        root = ""
        rel_fnames = ["non_existent_file"]
        addable_rel_fnames = []
        commands = None
        autocompleter = AutoCompleter(root, rel_fnames, addable_rel_fnames, commands)
        self.assertEqual(autocompleter.words, set())

if __name__ == "__main__":
    unittest.main()
>>>>>>> UPDATED
```

This new test creates an instance of `AutoCompleter` with a non-existent file and checks that `self.words` is an empty set.

tests/test_io.py

@@ -2,7 +2,7 @@ import os
 import unittest
 from unittest.mock import patch
 
-from aider.io import InputOutput
+from aider.io import InputOutput, AutoCompleter
 
 
 class TestInputOutput(unittest.TestCase):
@@ -11,6 +11,14 @@ class TestInputOutput(unittest.TestCase):
             io = InputOutput()
             self.assertFalse(io.pretty)
 
+    def test_autocompleter_with_non_existent_file(self):
+        root = ""
+        rel_fnames = ["non_existent_file"]
+        addable_rel_fnames = []
+        commands = None
+        autocompleter = AutoCompleter(root, rel_fnames, addable_rel_fnames, commands)
+        self.assertEqual(autocompleter.words, set())
+
 
 if __name__ == "__main__":
     unittest.main()