prevent npe with empty flow

git-svn-id: https://svn.apache.org/repos/asf/xmlgraphics/fop/trunk@195232 13f79535-47bb-0310-9956-ffa450edef68

src/org/apache/fop/render/AbstractRenderer.java

@@ -344,8 +344,9 @@ public abstract class AbstractRenderer extends AbstractLogEnabled
     protected void renderFlow(Flow flow) {
         // the normal flow reference area contains stacked blocks
         List blocks = flow.getChildAreas();
-        renderBlocks(blocks);
-
+        if(blocks != null) {
+            renderBlocks(blocks);
+        }
     }
 
     /**