Fix react 15 console warning: 'value' prop on forms should not be null

the value in redux store shouldn't be affected.
caused by small breaking change in react 15: facebook/react#5864

src/createField.js

@@ -33,7 +33,7 @@ const mapStateToProps = (state, props) => {
     isFieldEnabled: field.isEnabled,
     isValidating: field.isValidating,
     touched: field.touched,
-    value: field.value,
+    value: field.value || '',
   };
 };