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.