PR: Slab edge material not exported

### Problem Description

Similarly to #649, when exporting a slab edge that has a material assigned, the resulting entity has no material assigned to it.

Tested for IFC2x3 CV2 as well as IFC4 RVArch. Not a regression btw, happens in 23.2.5 as well.

Minimal model to reproduce: https://github.com/andydandy74/MyIssues/blob/master/RevitIFC650/133_IFC_NoSlabEdgeMaterial.rvt

### Revit Version

2023.1.x

### IFC for Revit Addon Version

23.3.0

### Windows Version

10 22H2

Start with the minimal model RevitIFC650/133_IFC_NoSlabEdgeMaterial.rvt and reproduce the export for IFC2x3 CV2 and IFC4 RVArch. Compare the slab edge material assignment in the Revit model with the resulting IFC entities; done means the exported slab edge retains its assigned material in both schemas.

Written by the indexing model from the issue text.