Fix coverage issue with Annotator::AnnotatorImpl::exists().
Fix coverage issue with Annotator::AnnotatorImpl::exists().

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