Fixed error editing pset with prop with empty prop.NominalValue

Traceback:
```
Error: Python: Traceback (most recent call last):
  File "\blenderbim\bim\module\pset\operator.py", line 103, in execute
    self.load_from_pset_data(pset)
  File "\blenderbim\bim\module\pset\operator.py", line 226, in load_from_pset_data
    value = prop.NominalValue.wrappedValue
AttributeError: 'NoneType' object has no attribute 'wrappedValue'
```

src/blenderbim/blenderbim/bim/module/pset/operator.py

@@ -223,7 +223,7 @@ class EnablePsetEditing(bpy.types.Operator):
                     new.is_selected = enum in selected_enum_items
             else:
                 if prop.is_a("IfcPropertySingleValue"):
-                    value = prop.NominalValue.wrappedValue
+                    value = prop.NominalValue.wrappedValue if prop.NominalValue else None
                 elif prop.is_a("IfcPhysicalSimpleQuantity"):
                     value = prop[3]
                 new_prop = self.props.properties.add()