Fix NPE when debouncing empty source (#6559)

Author: hluhovskyi

src/main/java/io/reactivex/internal/operators/flowable/FlowableDebounce.java

@@ -119,7 +119,9 @@ public void onComplete() {
             if (!DisposableHelper.isDisposed(d)) {
                 @SuppressWarnings("unchecked")
                 DebounceInnerSubscriber<T, U> dis = (DebounceInnerSubscriber<T, U>)d;
-                dis.emit();
+                if (dis != null) {
+                    dis.emit();
+                }
                 DisposableHelper.dispose(debouncer);
                 downstream.onComplete();
             }

src/main/java/io/reactivex/internal/operators/observable/ObservableDebounce.java

@@ -112,7 +112,9 @@ public void onComplete() {
             if (d != DisposableHelper.DISPOSED) {
                 @SuppressWarnings("unchecked")
                 DebounceInnerObserver<T, U> dis = (DebounceInnerObserver<T, U>)d;
-                dis.emit();
+                if (dis != null) {
+                    dis.emit();
+                }
                 DisposableHelper.dispose(debouncer);
                 downstream.onComplete();
             }

src/test/java/io/reactivex/internal/operators/flowable/FlowableDebounceTest.java

@@ -547,4 +547,14 @@ public void timedError() {
         .test()
         .assertFailure(TestException.class);
     }
+
+    @Test
+    public void debounceOnEmpty() {
+        Flowable.empty().debounce(new Function<Object, Publisher<Object>>() {
+            @Override
+            public Publisher<Object> apply(Object o) {
+                return Flowable.just(new Object());
+            }
+        }).subscribe();
+    }
 }

src/test/java/io/reactivex/internal/operators/observable/ObservableDebounceTest.java

@@ -506,4 +506,14 @@ public void timedError() {
         .test()
         .assertFailure(TestException.class);
     }
+
+    @Test
+    public void debounceOnEmpty() {
+        Observable.empty().debounce(new Function<Object, ObservableSource<Object>>() {
+            @Override
+            public ObservableSource<Object> apply(Object o) {
+                return Observable.just(new Object());
+            }
+        }).subscribe();
+    }
 }