src: use nullptr in node_buffer.cc

Signed-off-by: gengjiawen <technicalcute@gmail.com>

PR-URL: https://github.com/nodejs/node/pull/25820


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>