fix(widget-select): allow optional field to use min/max (#3175)

packages/netlify-cms-widget-select/src/SelectPreview.js

@@ -1,13 +1,26 @@
 import PropTypes from 'prop-types';
 import React from 'react';
+import { List } from 'immutable';
+import ImmutablePropTypes from 'react-immutable-proptypes';
 import { WidgetPreviewContainer } from 'netlify-cms-ui-default';
 
+const ListPreview = ({ values }) => (
+  <ul>
+    {values.map((value, idx) => (
+      <li key={idx}>{value}</li>
+    ))}
+  </ul>
+);
+
 const SelectPreview = ({ value }) => (
-  <WidgetPreviewContainer>{value ? value.toString() : null}</WidgetPreviewContainer>
+  <WidgetPreviewContainer>
+    {value && (List.isList(value) ? <ListPreview values={value} /> : value)}
+    {!value && null}
+  </WidgetPreviewContainer>
 );
 
 SelectPreview.propTypes = {
-  value: PropTypes.string,
+  value: PropTypes.oneOfType([PropTypes.string, ImmutablePropTypes.list]),
 };
 
 export default SelectPreview;