[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

先閱讀 issue 中描述的 StructVectorLoader load 函式以及與 StructVectorUnloader 的互動。重現建立空名稱 StructVector 的 round trip，然後確認載入的向量保留原始欄位名稱，而不是 ""。

由索引模型根據 Issue 內容生成。