Suppress future Error Prone error for AsyncFunction that returns a _n…

…ull Future_, rather than the intended _Future whose value is null_.

-------------
Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=86830267

guava-gwt/test-super/com/google/common/util/concurrent/super/com/google/common/util/concurrent/FuturesTest.java

@@ -673,6 +673,7 @@ public void testCatchingAsync_nullInsteadOfFuture() throws Exception {
     ListenableFuture<?> chainedFuture = Futures.catchingAsync(inputFuture, Throwable.class,
         new AsyncFunction<Throwable, Integer>() {
           @Override
+          @SuppressWarnings("AsyncFunctionReturnsNull")
           public ListenableFuture<Integer> apply(Throwable t) {
             return null;
           }

guava-tests/test/com/google/common/util/concurrent/FuturesTest.java

@@ -1039,6 +1039,7 @@ public void testCatchingAsync_nullInsteadOfFuture() throws Exception {
     ListenableFuture<?> chainedFuture = Futures.catchingAsync(inputFuture, Throwable.class,
         new AsyncFunction<Throwable, Integer>() {
           @Override
+          @SuppressWarnings("AsyncFunctionReturnsNull")
           public ListenableFuture<Integer> apply(Throwable t) {
             return null;
           }