Fix firstChild code to please clang-tidy.
Fix firstChild code to please clang-tidy.

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