fix handling of whitespace-only zippers. (fixes #20)

Since 'z/edn' is 'z/edn*' + moving to the first non-whitespace node,
handling whitespace-only cases is done by falling back to the latter
function's behaviour and having the zipper end up on the top-level
':forms' node.

Author: Yannick Scherer

src/rewrite_clj/zip/base.clj

@@ -22,9 +22,9 @@
    to the first non-whitespace/non-comment child."
   [node]
   (if (= (node/tag node) :forms)
-    (-> (edn* node)
-        (z/down)
-        (ws/skip-whitespace))
+    (let [top (edn* node)]
+      (or (-> top z/down ws/skip-whitespace)
+          top))
     (recur (node/forms-node [node]))))
 
 ;; ## Inspection