Avro adapter: AvroNullableConsumer should use readIndex

### Describe the bug, including details regarding any error messages, version, and platform.

When using Avro's `ValidatingDecoder`, any schema with a nullable field fails with the following exception:

```
Exception in thread "main" org.apache.avro.AvroTypeException: Attempt to process a int when a union was expected.
at org.apache.avro.io.parsing.Parser.advance(Parser.java:91)
at org.apache.avro.io.ValidatingDecoder.readInt(ValidatingDecoder.java:81)
at org.apache.arrow.adapter.avro.consumers.AvroNullableConsumer.consume(AvroNullableConsumer.java:44)
```

Reason: `AvroNullableConsumer` uses `readInt` to read the index, but it should use `readIndex` instead. It's irrelevant in case of a `BinaryDecoder`, for example (`readIndex` is the same as `readInt`), but `ValidatingDecoder`s and any Decoders based on that have different logic.

Commencez dans AvroNullableConsumer.consume, au niveau de l’appel à readInt identifié dans la stack trace, et comparez-le à l’API du decoder d’Avro ainsi qu’au comportement de ValidatingDecoder. Le travail est terminé lorsque les champs nullable peuvent être consommés avec un ValidatingDecoder sans l’exception union-versus-int.

Rédigé par le modèle d'indexation à partir du texte de l'issue.