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.

從 AvroNullableConsumer.consume 中 stack trace 所識別的 readInt 呼叫開始，並將其與 Avro 的 decoder API 和 ValidatingDecoder 的行為進行比較。當 nullable 欄位可以使用 ValidatingDecoder 消費且不會出現 union-versus-int 例外時，即表示完成。

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