Guard append_asset against an empty aggregate attribute (#7261)

get_tuple_type in project.append_asset descends nested tuples with
`while isinstance(tuple_, tuple): tuple_ = tuple_[0]`. When an aggregate
attribute is empty (e.g. an IfcCartesianPointList2D with CoordList == ((),))
the loop indexes [0] into an empty tuple and raises
"IndexError: tuple index out of range", aborting the whole append.

Stop descending at an empty tuple (`and tuple_`); it then returns `tuple` as
the element type, which matches neither entity_instance nor float in the
copy loop, so the empty aggregate is carried through unchanged and the
append proceeds.

Verified: appending an IfcArbitraryClosedProfileDef whose point list is
empty no longer raises and preserves the empty CoordList; a normal
populated profile appends unchanged.

Generated with the assistance of an AI coding tool.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/ifcopenshell-python/ifcopenshell/api/project/append_asset.py

@@ -775,7 +775,10 @@ class Usecase:
 
         # Utils method for the loop.
         def get_tuple_type(tuple_: tuple) -> type:
-            while isinstance(tuple_, tuple):
+            # Guard against empty (possibly nested) tuples, e.g. an aggregate
+            # attribute set to `()` or `((),)`, which would otherwise index
+            # into an empty tuple and raise IndexError (see #7261).
+            while isinstance(tuple_, tuple) and tuple_:
                 tuple_ = tuple_[0]
             return type(tuple_)