[Java] StructVectorLoader load function creates a vector with "" as name

### Describe the enhancement requested

`StructVectorLoader` `load` function does the following to create the resultant `StructVector`.

```java
StructVector result = StructVector.emptyWithDuplicates("", allocator);
```

Even though the buffers are properly re-created, it doesn't contain accurate field names, since the original vector's name cannot be recovered here. This would be an issue when the `StructVectorUnloader` and `StructVectorLoader` is used in a certain program.

### Component(s)

Java

首先阅读 issue 中描述的 StructVectorLoader 的 load 函数以及与 StructVectorUnloader 的交互。复现创建空名称 StructVector 的往返过程，然后验证已加载的向量保留原始字段名称，而不是 ""。

由索引模型根据 Issue 内容生成。