IfcCurveBoundedPlane : InnerBoundaries attribute is not optional

Despite what is said in ifc documentation description:
> An optional set of inner boundaries.

[IfcCurveBoundedPlane](https://standards.buildingsmart.org/IFC/RELEASE/IFC4/ADD2_TC1/HTML/link/ifccurveboundedplane.htm) `InnerBoundaries` attribute is not optional as specified in EXPRESS specifications:
> InnerBoundaries : SET [0:?] OF IfcCurve;

The `SET` can be empty but must exist as it is not prefixed with `OPTIONAL`. See https://github.com/IfcOpenShell/IfcOpenShell/issues/635#issuecomment-770376223

Example of current export from revit-ifc:
`#210= IFCCURVEBOUNDEDPLANE(#209,#204,$);`

This apply for both IFC2X3 and IFC4.

Start by tracing the revit-ifc export path for IfcCurveBoundedPlane and compare its output for IFC2X3 and IFC4 with the EXPRESS specification. Reproduce the current `#210= IFCCURVEBOUNDEDPLANE(#209,#204,$);` result and verify that the completed export represents the empty InnerBoundaries set as required.

Written by the indexing model from the issue text.