fix possible NPE introduced in #60591

x-pack/plugin/transform/src/main/java/org/elasticsearch/xpack/transform/transforms/pivot/CompositeBucketsChangeCollector.java

@@ -565,7 +565,7 @@ public class CompositeBucketsChangeCollector implements ChangeCollector {
                             entry.getKey(),
                             entry.getValue().getMissingBucket(),
                             ((DateHistogramGroupSource) entry.getValue()).getRounding(),
-                            entry.getValue().getField().equals(synchronizationField)
+                            entry.getValue().getField() != null && entry.getValue().getField().equals(synchronizationField)
                         )
                     );
                     break;