Consider unterminated f-strings in `FStringRanges` (#8154)
## Summary
This PR removes the `debug_assertion` in the `Indexer` to allow
unterminated f-strings. This is mainly a fix in the development build
which now matches the release build.
The fix is simple: remove the `debug_assertion` which means that the
there could be `FStringStart` and possibly `FStringMiddle` tokens
without a corresponding f-string range in the `Indexer`. This means that
the code requesting for the f-string index need to account for the
`None` case, making the code safer.
This also updates the code which queries the `FStringRanges` to account
for the `None` case. This will happen when the `FStringStart` /
`FStringMiddle` tokens are present but the `FStringEnd` token isn't
which means that the `Indexer` won't contain the range for that
f-string.
## Test Plan
`cargo test`
Taking the following code as an example:
```python
f"{123}
```
This only emits a `FStringStart` token, but no `FStringMiddle` or
`FStringEnd` tokens.
And,
```python
f"\.png${
```
This emits a `FStringStart` and `FStringMiddle` token, but no
`FStringEnd` token.
fixes: #8065
