meson: allow checking for null pointers even if they're supposed to be non-null
../src/c11/impl/time.c: In function 'timespec_get':
../src/c11/impl/time.c:71:8: error: 'nonnull' argument 'ts' compared to NULL [-Werror=nonnull-compare]
71 | if (!ts)
| ^
cc1: all warnings being treated as errors