New unit tests to check null argument in scaling factor
New unit tests to check null argument in scaling factor

nickerso/libcellml,MichaelClerx/libcellml,cellml/libcellml,hsorby/libcellml,MichaelClerx/libcellml,hsorby/libcellml,cellml/libcellml,nickerso/libcellml,hsorby/libcellml,MichaelClerx/libcellml,nickerso/libcellml,cellml/libcellml,cellml/libcellml,hsorby/libcellml,nickerso/libcellml