CLJS-2798: ChunkCons -next doesn't handle nil more

This revison aligns more with the Clojure implementation.

In the Clojure implemenentation, if _more is nil, then it would end up
doing a seq on an empty list, which would produce nil. This revision
simply pre-checks if more is nil, and if not, returns nil in that case.

The change also eliminates an unnecessary check of the return value of
-seq (checking for nil and returning nil), and just returns the value
-seq produces.

Author: mfikes

src/main/cljs/cljs/core.cljs

@@ -3553,9 +3553,8 @@ reduces them without incurring seq initialization"
   (-next [coll]
     (if (> (-count chunk) 1)
       (ChunkedCons. (-drop-first chunk) more meta nil)
-      (let [more (-seq more)]
-        (when-not (nil? more)
-          more))))
+      (when-not (nil? more)
+        (-seq more))))
 
   IChunkedSeq
   (-chunked-first [coll] chunk)

src/test/cljs/cljs/collections_test.cljs

@@ -1019,6 +1019,11 @@
       (is (not (map-entry? coll)))
       (is (= [:a 1] coll)))))
 
+(deftest test-cljs-2798
+  (is (nil? (let [b (chunk-buffer 1)]
+              (chunk-append b 0)
+              (next (chunk-cons (chunk b) nil))))))
+
 (comment
 
   (run-tests)