tests: libc: thrd: do not pass NULL for thrd_sleep() duration

The specification does not say that the first struct timespec passed to
thrd_sleep() may be NULL.

Preserving the testpoint so that a possible future testsuite can be
run that expects faults to occur when undefined behaviour is used.

Faults are one possible solution to handling undefined behaviour, but
it would be good to come to a concensus about how that should be
handled in Zephyr for standard function calls belonging to ISO C or
POSIX.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>