fix: transition SSR (#619)

* Support SSR

* Update src/Transition.js

* rm whitespace

Co-authored-by: Jason Quense <monastic.panic@gmail.com>

Author: Alicia Lopez

src/Transition.js

@@ -390,7 +390,11 @@ Transition.propTypes = {
    *     (see
    *     [test/CSSTransition-test.js](https://github.com/reactjs/react-transition-group/blob/13435f897b3ab71f6e19d724f145596f5910581c/test/CSSTransition-test.js#L362-L437)).
    */
-  nodeRef: PropTypes.shape({ current: PropTypes.instanceOf(Element) }),
+  nodeRef: PropTypes.shape({
+    current: typeof Element === 'undefined'
+      ? PropTypes.any
+      : PropTypes.instanceOf(Element)
+  }),
 
   /**
    * A `function` child can be used instead of a React element. This function is

test/SSR-test.js

@@ -0,0 +1,10 @@
+/**
+ * @jest-environment node
+ */
+
+// test that import does not crash
+import * as ReactTransitionGroup from '../src'; // eslint-disable-line no-unused-vars
+
+describe('SSR', () => {
+  it('should import react-transition-group in node env', () => {});
+});