tests: kernel: Change a mbox testcase that condition unsuitable

Fix issue #33114 #33120. Modify the testcase that run failed on iotdk
and nsim. This testing do not need receive thread ID when invoke
k_mbox_data_get() with NULL param. The testcase purpose is invoke
this API with NULL buffer and NULL receive_id. It will cause fatal
error if use a uninitialize receive id.

Signed-off-by: Jian Kang <jianx.kang@intel.com>