don't multiplex null keys to fix Combine.globally

runners/google-cloud-dataflow-java/src/main/java/org/apache/beam/runners/dataflow/internal/KeyedWindow.java

@@ -203,7 +203,6 @@ public void verifyCompatibility(WindowFn<?, ?> other) throws IncompatibleWindowE
       if (other instanceof KeyedWindowFn) {
         windowFn.verifyCompatibility(((KeyedWindowFn<?, ?, ?>) other).windowFn);
       }
-      ;
     }
 
     @Override

runners/google-cloud-dataflow-java/src/main/java/org/apache/beam/runners/dataflow/internal/StateMultiplexingGroupByKey.java

@@ -135,6 +135,12 @@ public void setup() {
                       @ProcessElement
                       public void processElement(ProcessContext c) {
                         KV<K, V> kv = c.element();
+                        if (kv.getKey() == null) {
+                          // Combine.globally treats null keys specially
+                          // so don't multiplex them.
+                          c.output(largeKeys, KV.of(null, kv.getValue()));
+                          return;
+                        }
                         try {
                           // clear output stream
                           byteStringOutputStream.toByteStringAndReset();