Undefined offset: 0 in openlayers_views_style_data->render()

When there is no data to render in a Data Overlay we get these messages:

Which can be made silent with a change like that:

diff --git a/modules/openlayers_views/views/openlayers_views_style_data.inc b/modules/openlayers_views/views/openlayers_views_style_data.inc
index 04850d5..ec0495b 100644
--- a/modules/openlayers_views/views/openlayers_views_style_data.inc
+++ b/modules/openlayers_views/views/openlayers_views_style_data.inc
@@ -33,9 +33,11 @@ class openlayers_views_style_data extends views_plugin_style {
       // If we are in preview mode, dump out some useful information about this data layer
       $output = "You can use the following parameters in your styles as dynamic values";
       $output .= "\n------------\n";
-      $keys = array_keys($data);
-      foreach ($data[$keys[0]]['attributes'] as $key => $value) {
-        $output .= '${'.$key."}\n";
+      if (!empty($data)) {
+        $keys = array_keys($data);
+        foreach ($data[$keys[0]]['attributes'] as $key => $value) {
+          $output .= '${'.$key."}\n";
+        }
       }
       $output .= "\n------------\n";