Update snippets head and last to type check

Update `head` and `last` to check on empty, undefined, null arrays.

snippets/head.md

@@ -5,12 +5,16 @@ tags: array,beginner
 
 Returns the head of a list.
 
-Use `arr[0]` to return the first element of the passed array.
+Check if `arr` is truthy and has a `length` property, use `arr[0]` if possible
+to return the first element, otherwise return `undefined`
 
 ```js
-const head = arr => arr[0];
+const head = arr => (arr && arr.length ? arr[0] : undefined);
 ```
 
 ```js
 head([1, 2, 3]); // 1
-```
+head([]); // undefined
+head(null); // undefined
+head(undefined); // undefined
+```

snippets/last.md

@@ -5,12 +5,17 @@ tags: array,beginner
 
 Returns the last element in an array.
 
-Use `arr.length - 1` to compute the index of the last element of the given array and returning it.
+Check if `arr` is truthy and has a `length` property, use `arr.length - 1` to
+compute the index of the last element of the given array and returning it,
+otherwise return `undefined`
 
 ```js
-const last = arr => arr[arr.length - 1];
+const last = arr => (arr && arr.length ? arr[arr.length - 1] : undefined);
 ```
 
 ```js
 last([1, 2, 3]); // 3
-```
+last([]); // undefined
+last(null); // undefined
+last(undefined); // undefined
+```