test(bigtable): suppress NullArgumentForNonNullParameter warnings

Suppress NullArgumentForNonNullParameter in EnhancedBigtableStubTest for
calls to assertThat(f.get()).isNull() where f.get() returns Void.

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();
@@ -389,6 +390,7 @@ public void testMutateRowRequestResponseConversion()
   }
 
   @Test
+  @SuppressWarnings("NullArgumentForNonNullParameter")
   public void testMutateRowRequestParams() throws ExecutionException, InterruptedException {
     RowMutation req = RowMutation.create(TABLE_ID, "my-key").deleteRow();