Fix implicit conversion to bool in model.cpp.
Fix implicit conversion to bool in model.cpp.

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