tests: counter_basic_api will return pass even if no counter is found

In test_all_instances(), if ARRAY_SIZE(devices) is 0, the test is not
performed. At this time, the test result should be TC_FAIL, instead of
TC_PASS. A zassert is needed to filter this case.
Fixes: #45647

Signed-off-by: Hu Zhenyu <zhenyu.hu@intel.com>