ReactTransitionGroup: Fix moving from falsey child

See http://stackoverflow.com/q/23510413/49485.

Test Plan: grunt fasttest

src/addons/transitions/ReactTransitionGroup.js

@@ -62,14 +62,16 @@ var ReactTransitionGroup = React.createClass({
 
     for (key in nextChildMapping) {
       var hasPrev = prevChildMapping && prevChildMapping.hasOwnProperty(key);
-      if (!hasPrev && !this.currentlyTransitioningKeys[key]) {
+      if (nextChildMapping[key] && !hasPrev &&
+          !this.currentlyTransitioningKeys[key]) {
         this.keysToEnter.push(key);
       }
     }
 
     for (key in prevChildMapping) {
       var hasNext = nextChildMapping && nextChildMapping.hasOwnProperty(key);
-      if (!hasNext && !this.currentlyTransitioningKeys[key]) {
+      if (prevChildMapping[key] && !hasNext &&
+          !this.currentlyTransitioningKeys[key]) {
         this.keysToLeave.push(key);
       }
     }

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

@@ -163,4 +163,22 @@ describe('ReactCSSTransitionGroup', function() {
     expect(a.getDOMNode().childNodes[0].id).toBe('one');
   });
 
+  it('should transition from false to one', function() {
+    var a = React.renderComponent(
+      <ReactCSSTransitionGroup transitionName="yolo">
+        {false}
+      </ReactCSSTransitionGroup>,
+      container
+    );
+    expect(a.getDOMNode().childNodes.length).toBe(0);
+    React.renderComponent(
+      <ReactCSSTransitionGroup transitionName="yolo">
+        <span key="one" id="one" />
+      </ReactCSSTransitionGroup>,
+      container
+    );
+    expect(a.getDOMNode().childNodes.length).toBe(1);
+    expect(a.getDOMNode().childNodes[0].id).toBe('one');
+  });
+
 });