Change default elephant tail to null (#25)

It was confusing to see

    tail... = undefined

in the struct definition and then

    if (tail == null)

later in the exercise - it appears that the mismatch would be the issue
- but that's distracting from the real issue: making the value optional!

Changing the initial value to null is still correct, but won't distract.

The only worry now is that the user will remember the undefined
definition from the previous exercise and wonder if that has to be that
way...but you can't win them all!

exercises/46_optionals2.zig

@@ -9,7 +9,7 @@ const std = @import("std"); // single quotes
 
 const Elephant = struct {
     letter: u8,
-    tail: *Elephant = undefined, // <---- make this optional!
+    tail: *Elephant = null, // <---- make this optional!
     visited: bool = false,
 };
 

patches/patches/46_optionals2.patch

@@ -1,11 +1,8 @@
 12c12
-<     tail: *Elephant = undefined, // <---- make this optional!
+<     tail: *Elephant = null, // <---- make this optional!
 ---
->     tail: ?*Elephant = undefined,
-39,42c39
-<         // We should stop once we encounter a tail that
-<         // does NOT point to another element. What can
-<         // we put here to make that happen?
+>     tail: ?*Elephant = null, // <---- make this optional!
+42c42
 <         if (e.tail == null) ???;
 ---
 >         if (e.tail == null) break;