saddle-points/test: Do not use head on empty list

[BassTony]

The second test case does not play well with head on line 21, since using head on an empty list yields an error.

Use head only when the list is not empty.

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[petertseng]

it's pretty hard to actually cause the test to actually evaluate head [], but just to demonstrate it's possible, you can make it happen with saddlePoints arr = if snd (snd (bounds arr)) == 3 then [(2, 1)] else []. Since there is a possible implementation that can make it happen, we should prevent it.