ReactTransitionGroup: Fix changing to null child

Fixes #1457.

Test Plan: grunt fasttest

src/addons/transitions/ReactTransitionGroup.js

@@ -61,15 +61,15 @@ var ReactTransitionGroup = React.createClass({
     var key;
 
     for (key in nextChildMapping) {
-      if (!prevChildMapping.hasOwnProperty(key) &&
-        !this.currentlyTransitioningKeys[key]) {
+      var hasPrev = prevChildMapping && prevChildMapping.hasOwnProperty(key);
+      if (!hasPrev && !this.currentlyTransitioningKeys[key]) {
         this.keysToEnter.push(key);
       }
     }
 
     for (key in prevChildMapping) {
-      if (!nextChildMapping.hasOwnProperty(key) &&
-        !this.currentlyTransitioningKeys[key]) {
+      var hasNext = nextChildMapping && nextChildMapping.hasOwnProperty(key);
+      if (!hasNext && !this.currentlyTransitioningKeys[key]) {
         this.keysToLeave.push(key);
       }
     }

src/addons/transitions/__tests__/ReactCSSTransitionGroup-test.js

@@ -142,4 +142,25 @@ describe('ReactCSSTransitionGroup', function() {
       container
     );
   });
+
+  it('should transition from one to null', function() {
+    var a = React.renderComponent(
+      <ReactCSSTransitionGroup transitionName="yolo">
+        <span key="one" id="one" />
+      </ReactCSSTransitionGroup>,
+      container
+    );
+    expect(a.getDOMNode().childNodes.length).toBe(1);
+    React.renderComponent(
+      <ReactCSSTransitionGroup transitionName="yolo">
+        {null}
+      </ReactCSSTransitionGroup>,
+      container
+    );
+    // (Here, we expect the original child to stick around but test that no
+    // exception is thrown)
+    expect(a.getDOMNode().childNodes.length).toBe(1);
+    expect(a.getDOMNode().childNodes[0].id).toBe('one');
+  });
+
 });