Fix bunch of error messages (commonly seen in IFCCSV) when you try to get a property value which is None

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

@@ -45,7 +45,7 @@ def get_properties(properties):
     results = {}
     for prop in properties:
         if prop.is_a("IfcPropertySingleValue"):
-            results[prop.Name] = prop.NominalValue.wrappedValue
+            results[prop.Name] = prop.NominalValue.wrappedValue if prop.NominalValue else None
         elif prop.is_a("IfcComplexProperty"):
             data = prop.get_info()
             data["properties"] = get_properties(prop.HasProperties)