[Bug] 自环节点作为source查询rays或rings报空指针异常

### Bug Type (问题类型)

other exception / error (其他异常报错)

### Before submit

- [X] 我已经确认现有的 [Issues](https://github.com/hugegraph/hugegraph/issues) 与 [FAQ](https://hugegraph.github.io/hugegraph-doc/guides/faq.html) 中没有相同 / 重复问题

### Environment (环境信息)

- Server Version: v0.11.2
- Backend: RocksDB
- Data Size: 3 vertices, 2 edges

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


http://localhost:8080/graphs/lineage/traversers/rays?source="bigdata_hive.null_test.nulltestb"&max_depth=10000&direction=IN&label=dependent&limit=100000

如图所示，对B节点请求traverser的rays报如下异常
{
"exception": "class java.lang.NullPointerException",
"message": "",
"cause": ""
}

### Vertex/Edge example (问题点 / 边数据举例)

_No response_

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

_No response_

Reproduce the failure against HugeGraph v0.11.2 with the provided three-vertex, two-edge dataset and the rays request for source "bigdata_hive.null_test.nulltestb". Compare the rays and rings traverser paths, using direction=IN, label=dependent, max_depth=10000, and limit=100000, then trace the resulting NullPointerException. Done means the self-loop source no longer returns a null pointer exception and the traversal returns a valid response.

Written by the indexing model from the issue text.