NPE when calling OnDiskGraphIndex#ramBytesUsed

Looks like http://github.com/datastax/jvector/pull/561 might have introduced a regression for the `OnDiskGraphIndex#ramBytesUsed` method. I am getting this NPE:

```
java.lang.NullPointerException: Cannot invoke "org.agrona.collections.Int2ObjectHashMap.values()" because "neighbors" is null
    at io.github.jbellis.jvector.graph.disk.OnDiskGraphIndex.ramBytesUsed(OnDiskGraphIndex.java:406)
```