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

…l (#15)

* 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

* disable eslint complexity

src/createField.js

@@ -17,6 +17,7 @@ import {
 import withForm from './withForm';
 import _createField from './_createField';
 
+/* eslint-disable complexity */
 const mapStateToProps = (state, props) => {
   const formId = props.form.getFormId();
   const fieldNames = getFieldNames(props.name);
@@ -33,9 +34,10 @@ const mapStateToProps = (state, props) => {
     isFieldEnabled: field.isEnabled,
     isValidating: field.isValidating,
     touched: field.touched,
-    value: field.value,
+    value: field.value || '',
   };
 };
+/* eslint-enable complexity */
 
 const mapDispatchToProps = (dispatch, props) => ({
   mergeFields: fieldNames => object => {