[bugfix] Avoid an NPE when there are no XUpdate Conditionals

Author: adamretter

exist-core/src/main/java/org/exist/xupdate/XUpdateProcessor.java

@@ -950,7 +950,7 @@ private Conditional popConditional() {
 
     private @Nullable Conditional peekConditional() {
         if (conditionals == null) {
-            throw null;
+            return null;
         }
         return conditionals.peek();
     }