Using LIMIT 1 is fundamentally wrong. This cuts off some of the correct answers in the case when there are several pairs of actors with the maximum number of meetings. In the tests, there is only one pair of actors with the maximum number of meetings. However, in real life, there may be several pairs - correct answers. So in general this solution is wrong.

The test cases just never set arr to None, that's why there's no error. What they do though is setting arr to [], which is different than None.
But in general you're right. One should check if arr is None before iterating over it.