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.wrapped_data.declaration().as_entity()
         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:

src/ifcopenshell-python/test/util/test_unit.py

@@ -123,6 +123,12 @@ class TestGetPropertyUnit(test.bootstrap.IFC4):
         prop.Unit = length2
         assert subject.get_property_unit(prop, self.file) == length2
 
+    def test_single_value_with_no_nominal_value(self):
+        # NominalValue is optional -- a property may be null. Must not crash.
+        ifcopenshell.api.root.create_entity(self.file, ifc_class="IfcProject")
+        prop = self.file.createIfcPropertySingleValue(Name="Foo", NominalValue=None)
+        assert subject.get_property_unit(prop, self.file) is None
+
     def test_enumerated_value(self):
         ifcopenshell.api.root.create_entity(self.file, ifc_class="IfcProject")
         length = ifcopenshell.api.unit.add_si_unit(self.file, unit_type="LENGTHUNIT", prefix="MILLI")
@@ -311,6 +317,13 @@ class TestGetUnitSymbol(test.bootstrap.IFC4):
         weird = ifcopenshell.api.unit.add_derived_unit(self.file, "USERDEFINED", "force per time", {force: 1, time: -1})
         assert subject.get_unit_symbol(weird) == "N/s"
 
+    def test_context_dependent_userdefined_unit_is_not_shadowed_by_the_derived_unit_check(self):
+        # IfcContextDependentUnit (e.g. "each", "boxes") is a distinct entity
+        # from IfcDerivedUnit, so the is_a("IfcDerivedUnit") check added for
+        # derived-unit symbol composition must not shadow this fallback.
+        each = ifcopenshell.api.unit.add_context_dependent_unit(self.file, name="EACH")
+        assert subject.get_unit_symbol(each) == "EACH"
+
 
 class TestIdentifyUnitDimensions(test.bootstrap.IFC4):
     def test_matches_a_named_unit_type_by_dimension(self):