fix NPE (#923)

Co-authored-by: Grahame Grieve <grahameg@gmail.ccom>

org.hl7.fhir.r5/src/main/java/org/hl7/fhir/r5/renderers/ValueSetRenderer.java

@@ -666,6 +666,9 @@ public class ValueSetRenderer extends TerminologyRenderer {
   }
 
   private  <T extends Resource> String getCsRef(T cs) {
+    if (cs == null) {
+      return "?cs-n?";
+    }
     String ref = (String) cs.getUserData("filename");
     if (ref == null)
       ref = (String) cs.getUserData("path");