Treat nodes with empty children arrays as leaves.

Fixes #286.

d3.layout.js

@@ -1027,12 +1027,12 @@ d3.layout.hierarchy = function() {
   // Also converts the data representation into a standard hierarchy structure.
   function recurse(data, depth, nodes) {
     var childs = children.call(hierarchy, data, depth),
+        n,
         node = d3_layout_hierarchyInline ? data : {data: data};
     node.depth = depth;
     nodes.push(node);
-    if (childs) {
+    if (childs && (n = childs.length)) {
       var i = -1,
-          n = childs.length,
           c = node.children = [],
           v = 0,
           j = depth + 1;

src/layout/hierarchy.js

@@ -7,12 +7,12 @@ d3.layout.hierarchy = function() {
   // Also converts the data representation into a standard hierarchy structure.
   function recurse(data, depth, nodes) {
     var childs = children.call(hierarchy, data, depth),
+        n,
         node = d3_layout_hierarchyInline ? data : {data: data};
     node.depth = depth;
     nodes.push(node);
-    if (childs) {
+    if (childs && (n = childs.length)) {
       var i = -1,
-          n = childs.length,
           c = node.children = [],
           v = 0,
           j = depth + 1;

test/layout/hierarchy-test.js

@@ -0,0 +1,21 @@
+require("../env");
+require("../../d3");
+require("../../d3.layout");
+
+var vows = require("vows"),
+    assert = require("assert");
+
+var suite = vows.describe("d3.layout.hierarchy");
+
+suite.addBatch({
+  "hierarchy": {
+    topic: d3.layout.hierarchy,
+    "doesn't overwrite the value of a node that has an empty children array": function(hierarchy) {
+      assert.deepEqual(hierarchy({value: 1, children: []}), [
+        {data: {value: 1, children: []}, depth: 0, value: 1}
+      ]);
+    }
+  }
+});
+
+suite.export(module);