[Java] UnionListWriter.writeVarChar get a empty string list

```
def writeListVector(writer: UnionListWriter, arr: Array[_], pose: Int): Unit = {
writer.startList()
writer.setPosition(pose)
arr.foreach {
case e: Short => writer.smallInt().writeSmallInt(e)
case e: Int => writer.integer().writeInt(e)
case e: Byte => writer.writeTinyInt(e)
case e: String =>
val rootAllocator = new RootAllocator()
val tempBuf = rootAllocator.buffer(2048)
val bytes = e.getBytes(StandardCharsets.UTF_8)
tempBuf.setBytes(0, bytes, 0, bytes.length)
writer.varChar().writeVarChar(0, bytes.length, tempBuf)
}
writer.setValueCount(arr.length)
writer.endList()
}
````
I use a function like above to write an array to a list vector, everything is normal except case String. It just returns a empty string list,which every elements' length is correct, like:
`[["", ""], [""], ["", "", ""]]`
can somebody help me?

### Component(s)

Java

Start by reproducing the example in writeListVector and tracing UnionListWriter.writeVarChar, focusing on why the output preserves element counts and lengths but loses string contents. Done means string elements written through the list vector are returned with their original values rather than empty strings.

Written by the indexing model from the issue text.