saddle-points/test: Do not use head on empty list (#1138)

The case with input `[]` does not work well with the 
`columns = length $ head xss`.

It's somewhat hard to have an implementation that actually causes
`head []` to be evalauted, but an example implementation that in fact
does is: 

```haskell
saddlePoints arr = if snd (snd (bounds arr)) == 3 then [(2, 1)] else []
```

Since there is a possible implementation that could cause `head []` to
be evaluated, we should rewrite it so that we never use `head` on an
empty list.

Author: BassTony

exercises/practice/saddle-points/test/Tests.hs

@@ -18,7 +18,7 @@ specs = describe "saddlePoints" $ for_ cases test
       where
         assertion = saddlePoints matrix `shouldBe` expected
         rows      = length xss
-        columns   = length $ head xss
+        columns   = if null xss then 0 else length $ head xss
         matrix    = listArray ((1, 1), (rows, columns)) (concat xss)
 
     cases = [ ( "Example from README",