[flake8-pie] Remove following comma correctly when the unpacked dictionary is empty (PIE800)

[InSyncWithFoo]

Summary

Resolves #15997.

Ruff used to introduce syntax errors while fixing these cases, but no longer will:

{"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.

[InSyncWithFoo]

The diff is best viewed with whitespace changes hidden.

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

[MichaReiser]

Thanks and hiding whitespace was an excellent suggestion!