Fix to sample code for href in ReactPropTypes

Error should be thrown in the previous condition is "not" meet. And the href propType is described as optional in the comment, but the null check was missing.

Author: protron

src/core/ReactPropTypes.js

@@ -55,7 +55,7 @@ var emptyFunction = require('emptyFunction');
  *      // An optional string or URI prop named "href".
  *      href: function(props, propName, componentName) {
  *        var propValue = props[propName];
- *        if (typeof propValue === 'string' || propValue instanceof URI) {
+ *        if (propValue != null && typeof propValue !== 'string' && !(propValue instanceof URI)) {
  *          return new Error(
  *            'Expected a string or an URI for ' + propName + ' in ' +
  *            componentName