MagicianInTraining tests don't check for insertion into an empty stack

Programming Swift
1

Magician-in-Training in Swift on Exercism exercise contains only a single test for the func insert(_ newCard: Int, atBottomOf stack: [Int]) -> [Int] function. While mentoring, I encountered a person, who wrote this code:

func insert(_ newCard: Int, atBottomOf stack: [Int]) -> [Int] {
    guard !stack.isEmpty else {
        return stack
    }
    var array = stack
    array.insert(newCard, at: 0)
    return array
}

The guard statement at the top is not just redundant, it actually prevents the function from executing with an empty stack, which is incorrect. But there’s no auto test to catch this, and so all tests pass.