stat-util: add explicit helpers for checking if stat/statx is initialized

stat-util: add explicit helpers for checking if stat/statx is initialized

This is a follow-up for 945a8210c770801c8492eda03b6e9af3ec5d03a3 and
makes the st_dev check generic, so that we can reuse it some other
places. It also incorporates the non-NULL check now, to be a
comprehensive one-stop solution.

The helper is static inline so that compilers can optimize the redundant
checks away in case it is combined with other checks.