Fix get_property_unit() crash on IfcPropertySingleValue.NominalValue = None

NominalValue is optional -- IfcPropertySingleValue permits a null value --
but get_property_unit() unconditionally accessed prop.NominalValue.is_a(),
crashing on any single-value property that's legitimately blank.

Also adds a regression test confirming IfcContextDependentUnit symbols
("each", "boxes", etc.) aren't shadowed by the IfcDerivedUnit branch added
in the previous commit.

src/ifcopenshell-python/ifcopenshell/util/unit.py

@@ -525,7 +525,8 @@ def get_property_unit(
         entity = prop.declaration
         measure_class = entity.attribute_by_index(3).type_of_attribute().declared_type().name()
     elif prop.is_a("IfcPropertySingleValue"):
-        measure_class = prop.NominalValue.is_a()
+        if value := prop.NominalValue:
+            measure_class = value.is_a()
     elif prop.is_a("IfcPropertyEnumeratedValue"):
         if prop.EnumerationReference:
             if unit := prop.EnumerationReference.Unit: