perf: don't reflow when there's no class to add

Avoid an unnecessary reflow in the CSSTransition component if the class
name is falsy.

Author: michaelkix

src/CSSTransition.js

@@ -210,10 +210,12 @@ class CSSTransition extends React.Component {
   reflowAndAddClass(node, className) {
     // This is for to force a repaint,
     // which is necessary in order to transition styles when adding a class name.
-    /* eslint-disable no-unused-expressions */
-    node && node.scrollTop;
-    /* eslint-enable no-unused-expressions */
-    addClass(node, className);
+    if (className) {
+      /* eslint-disable no-unused-expressions */
+      node && node.scrollTop;
+      /* eslint-enable no-unused-expressions */
+      addClass(node, className);
+    }
   }
 
   render() {