in is_null() example.
fix mistake doc example
in `is_null()` example.
In general, we don't take pull requests into the branch 'master'. We pull into 'develop', and then merge into master.
I've made your requested change on develop, and you should verify it there. If you're happy with it, let me know, and I will merge to master, where it will become part of the next release.
Sorry, I confirmed the change in develop branch.