building search response should handle cases where the sort is null valued

modules/elasticsearch/src/main/java/org/elasticsearch/common/xcontent/XContentBuilder.java

@@ -462,6 +462,9 @@ public final class XContentBuilder {
     }
 
     public XContentBuilder value(Object value) throws IOException {
+        if (value == null) {
+            return nullValue();
+        }
         Class type = value.getClass();
         if (type == String.class) {
             value((String) value);

modules/elasticsearch/src/main/java/org/elasticsearch/rest/action/search/RestSearchAction.java

@@ -97,6 +97,9 @@ public class RestSearchAction extends BaseRestHandler {
                     builder.endObject();
                     channel.sendResponse(new XContentRestResponse(request, OK, builder));
                 } catch (Exception e) {
+                    if (logger.isDebugEnabled()) {
+                        logger.debug("failed to execute search (building response)", e);
+                    }
                     onFailure(e);
                 }
             }