AWS: update DynamoDbCatalog.dropNamespace attr check

[bijanhoule]

This PR is intended to resolve an issue with DynamoDbCatalog.dropNamespace() -- the dynamoDB delete fails as the conditionExpression evaluates to e.g. attribute_exists(my_db), rather than attribute_exists(namespace).

Example namespace record in dynamo:

{
 "identifier": "NAMESPACE",
 "namespace": "my_db",
 "updated_at": 1628190933466,
 "created_at": 1628190933466,
 "v": "0203ec48-317b-4197-9712-4a7c91b5c91a",
 "p.owner": "bijan"
}

@jackye1995 - can you confirm this change makes sense / is what was intended?
@danielcweeks

[jackye1995]

can you also add the following integration test?

  @Test
  public void testDropNamespace() {
    Namespace namespace = Namespace.of(genRandomName());
    catalog.createNamespace(namespace);
    catalog.dropNamespace(namespace);
    GetItemResponse response = dynamo.getItem(GetItemRequest.builder()
        .tableName(catalogTableName)
        .key(DynamoDbCatalog.namespacePrimaryKey(namespace))
        .build());
    Assert.assertFalse("namespace must not exist", response.hasItem());
  }

[bijanhoule]

can you also add the following integration test?

...

Good call, thank you! Added

[rdblue]

Thanks for fixing this, @bijanhoule!