[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

Start by reading the StructVectorLoader load function and the StructVectorUnloader interaction described in the issue. Reproduce the round trip that creates a StructVector with an empty name, then verify that the loaded vector retains the original field name instead of "".

Written by the indexing model from the issue text.