fix(iohandler): preserve empty containers and literal False in template args (#6728)

[MyDude92]

Closes #6728

📑 Description

IOHandler's template-function parser previously dropped empty containers ([], {}, ()) and boolean False positional arguments due to an overly restrictive truthiness guard. This caused subsequent arguments to shift left, leading to parameter mismatch or unexpected TypeError exceptions during workflow evaluation.

Changes Made

1. keep/iohandler/iohandler.py: Replaced flawed check with if _arg is not None:. Ensures all valid evaluated arguments ("", 0, 0.0, False, [], {}, ()) are preserved positionally.
2. tests/test_iohandler.py: Added unit test test_iohandler_does_not_drop_falsy_arguments verifying keep.len and keep.eq on empty lists, dicts, tuples, and boolean False.

✅ Checks

• My pull request adheres to the code style of this project
• My code requires changes to the documentation
• I have updated the documentation as required
• All the tests have passed

ℹ Additional Information

Fixes parameter shifting and unexpected TypeErrors when workflows pass falsy collections or boolean False into keep.* functions.