fix(react): do not throw error when no options are passed

Currently, if you do not pass any options, the following error is thrown:

`TypeError: Cannot read property extensions of undefined`
This commit makes options an object by default, preventing the error

packages/react/lib/common.js

@@ -5,6 +5,7 @@ exports.createCombinedContext = function(
   ReactContext,
   combineContexts
 ) {
+  config = config || {};
   var mergedConfig = Object.assign({}, config);
   delete mergedConfig.extensions;
   var mergedContexts = [ReactContext];