fix(iohandler): don't drop empty list/dict/tuple args to keep.* functions

[Prabal864]

What

Fixes #6728.

IOHandler's keep.* template-function-call parser built its positional-argument list with:

if (
    _arg
    or _arg == ""
    or (_arg == 0 or _arg == 0.0)
    and _arg is not False
):
    _args.append(_arg)

The comment above it says this exists so "" and 0/0.0 aren't dropped as "empty" — but it only special-cases those two:

• A literal []/{}/() argument (parsed a few lines above via ast.literal_eval into a real, empty container) is falsy, not == "", and not == 0/0.0, so it fails every branch and gets silently dropped.
• A real (non-string) False — which can come back from a nested keep.* call — also gets dropped: False == 0 is True in Python, so (_arg == 0 or _arg == 0.0) is True, but the trailing and _arg is not False then makes the whole condition False.

Dropping an argument shifts every subsequent positional argument one slot left, either raising a confusing TypeError deep inside keep.functions, or silently binding the wrong value to the wrong parameter.

Fix

_arg is initialized to None and every parsing branch always reassigns it to a real value — it only stays None when nothing was parsed (e.g. a nested keep.* call that returned nothing). So the guard can simply be if _arg is not None:, which keeps everything the old guard kept ("", 0, 0.0) plus [], {}, (), and False, and only drops the genuine no-value case.

Testing

Added 3 cases to tests/test_iohandler.py::test_with_function_empty_container_argument, exercised through the real IOHandler.render():

• keep.join([], '-') → "" (previously the [] was dropped, leaving join('-'), which treats '-' as the iterable string and fails to parse it as JSON)
• keep.join({}, '-') → ""
• keep.len([]) → 0

Same environment limitation as my last two PRs here (#6720, #6726): poetry install fails on this platform because uvloop doesn't support Windows, so I couldn't run the full test suite. What I did verify:

• python -m py_compile passes on both changed files.
• black --check and isort --check report no new issues introduced by this diff (I confirmed the two pre-existing formatting complaints elsewhere in iohandler.py and test_iohandler.py are unrelated to and untouched by this change).
• Extracted the exact guard, copied verbatim, and ran it standalone against real ast-parsed [], {}, (), and non-empty-list arguments — confirmed the empty containers vanish under the old guard and are preserved under the new one, with no change to the already-correct cases.

A maintainer running this on Linux/macOS should see tests/test_iohandler.py::test_with_function_empty_container_argument pass outright.

Checklist

• I have read the Contributing Guide
• If you've added code that should be tested, add tests.
• If you've changed APIs, update the documentation. — not applicable, no API change.
• Ensure the test suite passes.
• Make sure your code lints (black/isort) — for the lines this PR actually touches.