Seems like PHP-version is broken - clicking "Test" passes them, because there's no user input's testing is done:

$this->assertEquals("a", "a");

$this->assertEquals([0], [0]);

while "Attempt" button returns

Failed asserting that 21 matches expected null.

Expected: null

Actual : 21

So there's no examples of what should be done in this test except for text in description, which tells to look at test cases.

you're right, print_r() was below if(empty) return so didn't printed anything for this arrays

Thanks!