[Java] Avro to arrow conversion fails for nested unions

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

Hello arrow team,

I believe the Avro adapter fails on avro union types nested in records and arrays. The allocator tries to allocate increasingly more memory until it reaches the configured max (ie you can use `-Darrow.vector.max_allocation_bytes=8589934592` to tell the allocator to use 1GB and it will try to allocate all of it for an empty array field with a nested union type field).

`Memory required for vector capacity 508400 is (2097152), which is more than max allowed (1048576)`

I believe this is because the while loop [here](https://github.com/apache/arrow/blob/main/java/adapter/avro/src/main/java/org/apache/arrow/adapter/avro/consumers/AvroStructConsumer.java#L69-L75) never terminates as `UnionVector::getValueCapacity` always returns 0 when the union is nested in either a record or an array.

I have a branch [here](https://github.com/darcysaum-toast/arrow/tree/avro-nested-union) with a [failing test](https://github.com/darcysaum-toast/arrow/blob/avro-nested-union/java/adapter/avro/src/test/java/org/apache/arrow/adapter/avro/AvroToArrowIteratorTest.java#L171).
```
@Test
public void testNestedUnion() throws Exception {
Schema schema = getSchema("test_nested_union.avsc");
Schema topChildSchema = schema
.getField("f0")
.schema();
GenericRecord parent = new GenericData.Record(schema);
GenericRecord topChild = new GenericData.Record(topChildSchema);

topChild.put("fNestedUnion", 1);
parent.put("f0", topChild);

List roots = new ArrayList<>();
try (AvroToArrowVectorIterator iterator = convert(schema, singletonList(parent))) {
while (iterator.hasNext()) {
roots.add(iterator.next());
System.out.println(roots.get(roots.size() - 1).contentToTSVString());
}
}
}
```

The [schema](https://github.com/darcysaum-toast/arrow/blob/avro-nested-union/java/adapter/avro/src/test/resources/schema/test_nested_union.avsc) is as follows:
```
{
"namespace": "org.apache.arrow.avro",
"type": "record",
"name": "testArrayOfRecords",
"fields": [
{
"name": "f0",
"type": {
"type": "record",
"name": "NestedRecord",
"fields": [
{
"name": "fNestedUnion",
"type": ["null", "int"],
"default": null
}
]
}
}
]
```

Changing type of field `fNestedUnion` from `["null", "int"]` to `"int"` will make the test pass (and the iterator correctly prints out the arrow data).

### Component(s)

Java

先从 java/adapter/avro/src/main/java/org/apache/arrow/adapter/avro/consumers/AvroStructConsumer.java 中第 69-75 行附近的分配循环开始，然后使用 schema/test_nested_union.avsc 运行 AvroToArrowIteratorTest.java 中失败的测试。完成的标准是嵌套 union 转换在不耗尽分配器的情况下完成，并且迭代器生成预期的 Arrow 数据。

由索引模型根据 Issue 内容生成。