Remove the children array for childless nodes.

If data is updated so that some nodes no longer have children, and a
custom children accessor is used, then the children array was not being
removed for such nodes, which meant that hierarchical layouts were
incorrectly taking into account these children.

Fixes d3#1579.

d3.js

@@ -6118,8 +6118,11 @@ d3 = function() {
         }
         if (sort) c.sort(sort);
         if (value) node.value = v;
-      } else if (value) {
-        node.value = +value.call(hierarchy, node, depth) || 0;
+      } else {
+        delete node.children;
+        if (value) {
+          node.value = +value.call(hierarchy, node, depth) || 0;
+        }
       }
       return node;
     }

src/layout/hierarchy.js

@@ -28,8 +28,11 @@ d3.layout.hierarchy = function() {
       }
       if (sort) c.sort(sort);
       if (value) node.value = v;
-    } else if (value) {
-      node.value = +value.call(hierarchy, node, depth) || 0;
+    } else {
+      delete node.children;
+      if (value) {
+        node.value = +value.call(hierarchy, node, depth) || 0;
+      }
     }
     return node;
   }

test/layout/hierarchy-test.js

@@ -20,6 +20,12 @@ suite.addBatch({
       assert.equal(nodes[0].value, 0);
       h.nodes(nodes[0]);
       assert.equal(nodes[0].value, 0);
+    },
+    "removes the children array for a node that has no children": function(hierarchy) {
+      var h = hierarchy(),
+          nodes = h.children(function() { return null; }).nodes({children: [{}]});
+      assert.equal(nodes[0].value, 0);
+      assert.isUndefined(nodes[0].children);
     }
   }
 });