## Summary
Resolves#15997.
Ruff used to introduce syntax errors while fixing these cases, but no
longer will:
```python
{"a": [], **{},}
# ^^^^ Removed, leaving two contiguous commas
{"a": [], **({})}
# ^^^^^ Removed, leaving a stray closing parentheses
```
Previously, the function would take a shortcut if the unpacked
dictionary is empty; now, both cases are handled using the same logic
introduced in #15394. This change slightly modifies that logic to also
remove the first comma following the dictionary, if and only if it is
empty.
## Test Plan
`cargo nextest run` and `cargo insta test`.
## Summary
This adds an autofix for PIE800 (unnecessary spread) -- whenever we see
a `**{...}` inside another dictionary literal, just delete the `**{` and
`}` to inline the key-value pairs. So `{"a": "b", **{"c": "d"}}` becomes
just `{"a": "b", "c": "d"}`.
I have enabled this just for preview mode.
## Test Plan
Updated the preview snapshot test.
