Add test for passing nullptr to hasEquivalentVariable.
Add test for passing nullptr to hasEquivalentVariable.

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