testsuite: ztress: Remove idle thread NULL check
testsuite: ztress: Remove idle thread NULL check

This commit removes the NULL check on the idle thread array
(`idle_tid`) because it is statically declared and will never be NULL.

This fixes the "comparison will always evaluate as ‘false’" warning
generated by the GCC 12.

Signed-off-by: Stephanos Ioannidis <dc76e9f0c0006e8f919e0c515c66dbba3982f785@stephanos.io>

finikorg/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr