Part: Create CompSolid/Compound/Shell if null before adding a sub-shape

Creating a shape with Part.CompSolid(), Part.Compound() or Part.Shell() is a null shape. In order to allow to append sub-shapes with the add()
method a valid shape must be created beforehand.

This fixes #13208

src/Mod/Part/App/TopoShapeCompSolidPyImp.cpp

@@ -25,6 +25,7 @@
 #ifndef _PreComp_
 # include <BRep_Builder.hxx>
 # include <Standard_Failure.hxx>
+# include <TopoDS.hxx>
 # include <TopoDS_CompSolid.hxx>
 #endif
 
@@ -109,6 +110,9 @@ PyObject* TopoShapeCompSolidPy::add(PyObject* args)
 
     BRep_Builder builder;
     TopoDS_Shape comp = getTopoShapePtr()->getShape();
+    if (comp.IsNull()) {
+        builder.MakeCompSolid(TopoDS::CompSolid(comp));
+    }
     auto shapes = getPyShapes(obj);
 
     try {