test(bigtable): restore f.get() verification and suppress warnings

Restore assertThat(f.get()).isNull() assertions in EnhancedBigtableStubTest
based on PR review feedback to explicitly document asynchronous future completion.
Add @SuppressWarnings("NullArgumentForNonNullParameter") annotations to resolve
Error Prone warnings triggered by Truth.assertThat() receiving an ApiFuture<Void>
result.

BUG=b/481669998
TAG=agy
CONV=7b193e4d-38f8-450c-8475-0b9ae0d04507

Author: lqiu96

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

@@ -371,6 +371,7 @@ public void testRMWRequestResponseConversion() throws ExecutionException, Interr
   }
 
   @Test
+  @SuppressWarnings("NullArgumentForNonNullParameter")
   public void testMutateRowRequestResponseConversion()
       throws ExecutionException, InterruptedException {
     RowMutation req = RowMutation.create(TableId.of("my-table"), "my-key").deleteRow();
@@ -385,9 +386,11 @@ public void testMutateRowRequestResponseConversion()
     MutateRowRequest protoReq = fakeDataService.mutateRowRequests.poll(1, TimeUnit.SECONDS);
     assertThat(protoReq)
         .isEqualTo(req.toProto(RequestContext.create(PROJECT_ID, INSTANCE_ID, APP_PROFILE_ID)));
+    assertThat(f.get()).isNull();
   }
 
   @Test
+  @SuppressWarnings("NullArgumentForNonNullParameter")
   public void testMutateRowRequestParams() throws ExecutionException, InterruptedException {
     RowMutation req = RowMutation.create(TABLE_ID, "my-key").deleteRow();
 
@@ -405,6 +408,8 @@ public void testMutateRowRequestParams() throws ExecutionException, InterruptedE
 
     // StatsHeadersUnaryCallable
     assertThat(reqMetadata.keys()).contains("bigtable-client-attempt-epoch-usec");
+
+    assertThat(f.get()).isNull();
   }
 
   @Test