fix: handle empty files in last_n_bytes_of

[immanuwell]

Bug

glint checks the final newline through envoy.base.utils.last_n_bytes_of().
On a 0-byte file that helper does a negative seek and blows up with OSError: [Errno 22] Invalid argument, so the check crashes instead of flagging the file. Empty files are normal repo inputs, so this edge case is pretty legit.

Fix

Clamp the seek offset to 0. Empty and short files now return the available tail bytes, nice and boring.

Repro

Before this change:

python - <<'PY'
from pathlib import Path
from tempfile import TemporaryDirectory
from envoy.code.check.abstract.glint import NewlineChecker

with TemporaryDirectory() as tmp:
    root = Path(tmp)
    empty = root / "empty.txt"
    empty.write_bytes(b"")
    print(NewlineChecker(root).no_newlines([empty.name]))
PY

This raised OSError: [Errno 22] Invalid argument.

After this change the same snippet returns {'empty.txt'}.

Checks

pytest -q py/envoy.base.utils/tests/test_utils.py -k 'last_n_bytes_of or ellipsize or is_sha'
pytest -q py/envoy.code.check/tests/test_abstract_glint.py -k 'newline_checker_no_newlines or glint_no_newlines or files_with_no_newline'

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