to_compound: return an empty compound for an empty list instead of raising (#600)

[youdie006]

Fixes #600.

Problem

to_compound() type-checks list inputs by indexing obj[0] before it handles the empty-list case:

elif isinstance(obj, list) and isinstance(obj[0], cq.Workplane):

So to_compound([]) raises IndexError: list index out of range. This is on a live render path: cq_editor/widgets/object_tree.py calls to_compound(node.obj).moved(world), so an empty selection or an empty intermediate result crashes the object-tree/assembly render.

Fix

Handle an empty list explicitly and return an empty compound, which is consistent with how an empty Workplane (whose vals() is empty) already flows through cq.Compound.makeCompound([]). I confirmed cq.Compound.makeCompound([]) returns an empty Compound rather than raising.

elif isinstance(obj, list) and not obj:
    pass  # an empty list produces an empty compound

(The issue left the choice open between an empty compound and a raised error; empty compound keeps to_compound total over its declared input types and matches the empty-Workplane behavior. Happy to switch to a ValueError if you'd prefer.)

Test

Added test_to_compound_empty_list in tests/test_cq_utils.py. Red-green verified with pytest tests/test_cq_utils.py: before the change the test raises IndexError; after, to_compound([]) returns an empty cq.Compound. The existing test_cq_utils.py test still passes.

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Disclosure: I used AI assistance (Claude) while preparing this change. I reproduced the crash, ran the tests (red-green), and take responsibility for the contribution.

[jmwright]

Thanks @youdie006