fix: pass falsy args to keep.* template functions

[DragonBot00]

Problem

IOHandler._parse_token silently drops falsy-but-valid arguments to keep.* template functions. When a function call resolves to [], {}, (), False, or 0, the argument is silently dropped instead of being passed to the function.

The current condition handles "", 0, and 0.0 but still drops [], {}, (), and False due to short-circuit evaluation.

Fix

Replace the complex condition with if _arg is not None:.

_arg is initialized to None and stays None only when no parsing branch matched (no real value produced). All other values including "", 0, False, [], {}, () are valid parsed results that should be passed through.

Testing

The edge cases that were previously dropped:

• Empty list
• Empty dict
• Empty tuple
• Boolean False
• Zero

Fixes #6728