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/TopoShapeShellPyImp.cpp

@@ -140,6 +140,9 @@ PyObject*  TopoShapeShellPy::add(PyObject *args)
 
     BRep_Builder builder;
     TopoDS_Shape shell = getTopoShapePtr()->getShape();
+    if (shell.IsNull()) {
+        builder.MakeShell(TopoDS::Shell(shell));
+    }
 
     try {
         const TopoShape& shape = *static_cast<TopoShapeFacePy*>(obj)->getTopoShapePtr();