fix(bigtable): resolve NullArgumentForNonNullParameter error in MaybePointWriteCallableTest

Fixes #14174

Replace redundant Truth assertThat((Object) future.get()).isNull() on ApiFuture<Void>
with direct future.get() to avoid ErrorProne NullArgumentForNonNullParameter false-positives
under GraalVM / JDK 21 CI. Also assert returned future in multipleEntries_fallsThroughToClassic
to satisfy FutureReturnValueIgnored.

Author: lqiu96

java-bigtable/google-cloud-bigtable/src/test/java/com/google/cloud/bigtable/data/v2/stub/mutaterows/MaybePointWriteCallableTest.java

@@ -56,7 +56,7 @@ public void singleEntry_routesToPointWriter() throws Exception {
     ApiFuture<Void> future = callable.futureCall(request, null);
     pointWriter.response.set(null);
 
-    assertThat((Object) future.get()).isNull();
+    future.get();
     assertThat(classic.request).isNull();
     assertThat(pointWriter.request).isNotNull();
     // The single entry is converted back into a RowMutation targeting the same row.
@@ -70,8 +70,9 @@ public void multipleEntries_fallsThroughToClassic() {
             .add("row-a", Mutation.create().deleteRow())
             .add("row-b", Mutation.create().deleteRow());
 
-    callable.futureCall(request, null);
+    ApiFuture<Void> future = callable.futureCall(request, null);
 
+    assertThat(future).isSameInstanceAs(classic.response);
     assertThat(pointWriter.request).isNull();
     assertThat(classic.request).isEqualTo(request);
   }