Fix NPE in PerFieldKnnVectorsFormat.getGraph

[dweiss]

Fixes occasional test failures:

Stack Trace:
java.lang.NullPointerException: Cannot read field "number" because "info" is null
        at __randomizedtesting.SeedInfo.seed([7F1D9331344693F5:8AA61B73099B4704]:0)
        at org.apache.lucene.codecs.perfield.PerFieldKnnVectorsFormat$FieldsReader.getGraph(PerFieldKnnVectorsFormat.java:322)
        at org.apache.lucene.index.TestKnnGraph.assertConsistentGraph(TestKnnGraph.java:385)
        at org.apache.lucene.index.TestKnnGraph.testMultipleVectorFields(TestKnnGraph.java:193)

[github-actions]

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