Fix OnDiskGraphIndex#ramBytesUsed NPE

[michaeljmarshall]

Fixes: #586

Now that we lazily load the inMemoryNeighbors and the inMemoryFeatures, we need to handle the case where they are null or have values that are null. The added test fails without the change.

[michaeljmarshall]

Supercedes #587

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[marianotepper]

LGTM