Fixed bug in IFC when dimensional exponent parameters in IfcSIUnits is not defined.

Usually, the value of this parameter is derived from parents, but it is not always the case for some files.

code/IFCReaderGen1.cpp

@@ -2652,6 +2652,7 @@ template <> size_t GenericFill<IfcNamedUnit>(const DB& db, const LIST& params, I
 	size_t base = 0;
 	if (params.GetSize() < 2) { throw STEP::TypeError("expected 2 arguments to IfcNamedUnit"); }    do { // convert the 'Dimensions' argument
         std::shared_ptr<const DataType> arg = params[base++];
+		if (dynamic_cast<const UNSET*>(&*arg)) break;
         if (dynamic_cast<const ISDERIVED*>(&*arg)) { in->ObjectHelper<Assimp::IFC::IfcNamedUnit,2>::aux_is_derived[0]=true; break; }
         try { GenericConvert( in->Dimensions, arg, db ); break; } 
         catch (const TypeError& t) { throw TypeError(t.what() + std::string(" - expected argument 0 to IfcNamedUnit to be a `IfcDimensionalExponents`")); }