Fix vtkXMLUnstructuredGridReader not importing .vtu with NULL cell array

Fix issue #19349:
A freshly initialized unstructured grid may be written without DataArray child
nodes in the Point and Cells elements but vtkXMLUnstructuredGridReader and
vtkXMLUnstructuredDataReader always expected them to have child nodes.
Made it only an error if the number of point or cells is not 0, respectively.
Added test case.