[Bug] Unit tests fail with NPE in HDFS-Tests (loader)

### Bug Type (问题类型)

None

### Before submit

- [x] I had searched in the [issues](https://github.com/apache/hugegraph-toolchain/issues) and found no similar issues.

### Environment (环境信息)

- Toolchain Version: v1.7.0

### Expected & Actual behavior (期望与实际表现)

refer: #704, when changing the default loader optional `--parser-threads` > 1

Running UT triggers a NullPointerException (NPE) in `testMultiFilesHaveHeader` and `testFilterPathBySuffix` within `HDFSLoaderTest`. (Currently set it to 1)

### Vertex/Edge example (问题点 / 边数据举例)

```javascript

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### Schema [VertexLabel, EdgeLabel, IndexLabel] (元数据结构)

```javascript

```

Start with HDFSLoaderTest, focusing on testMultiFilesHaveHeader and testFilterPathBySuffix, then review the parser-threads change referenced in #704. Run the HDFS tests with the default parser-threads value greater than 1 and trace the NullPointerException. Done means both tests pass without an NPE under that setting.

Written by the indexing model from the issue text.