Return an empty object when skipkeys drops every key

[GreedyC]

With skipkeys=True and indent=2, a mapping whose keys are all unsupported is serialized as {\n ,\n}, which cannot be parsed as JSON5.

Return {} when no keys were emitted. The regression covers numeric/string indentation, both trailing-comma settings, and an empty filtered object nested in an array.

Validation on Python 3.14.6: all 81 unit tests, doctests, Ruff, Pylint, Mypy, 100% line coverage, wheel/sdist builds, and Twine checks pass. The regression fails on the unchanged implementation. ./run presubmit exits successfully, but still prints an existing pyproject.toml formatting warning; the same warning was reproduced from the unchanged HEAD file. This PR does not alter that file.

[dpranke]

Good catch. Thanks for the fix!