Analyser: reworked some code to get 100% coverage in `Analyser::AnalyserImpl::analyseModel()`.
Analyser: reworked some code to get 100% coverage in `Analyser::AnalyserImpl::analyseModel()`.

Indeed, we were checking whether a @c Variable object was equal to `nullptr`. However, it's a bit tricky to create a test for this, so much easier to test whether it's not equal to `nullptr` instead.

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