Fallback to end-of-file if ends in trailing continuation (#6789)
## Summary
Given:
```python
def end_of_file():
if False:
return 1
x = 2 \
```
Then when searching for the end of the `x = 2` statement, we'd reach a
panic as we'd hit the last line (`\\`) and abort, since the universal
iterator doesn't return trailing newlines. Instead, we should just use
the end of the file as the fallback.
Closes https://github.com/astral-sh/ruff/issues/6787.
## Test Plan
`cargo test`
