Small fix for assertConsistentGraph (#631)

TestKnnGraph.testMultipleVectorFields sometimes breaks with
the following message:

java.lang.NullPointerException: Cannot invoke
 "org.apache.lucene.codecs.lucene91.Lucene91HnswVectorsReader.getGraphValues(String)"
because "vectorReader" is null

This happens in assertConsistentGraph.

This patch ensures that for a segment and a field  where there is no
vectors indexed, we don't run a check on consistent graph.

Author: mayya-sharipova

lucene/core/src/test/org/apache/lucene/index/TestKnnGraph.java

@@ -172,9 +172,11 @@ public void testMultipleVectorFields() throws Exception {
     int numDoc = atLeast(100);
     int[] dims = new int[numVectorFields];
     float[][][] values = new float[numVectorFields][][];
+    FieldType[] fieldTypes = new FieldType[numVectorFields];
     for (int field = 0; field < numVectorFields; field++) {
       dims[field] = atLeast(3);
       values[field] = randomVectors(numDoc, dims[field]);
+      fieldTypes[field] = KnnVectorField.createFieldType(dims[field], similarityFunction);
     }
 
     try (Directory dir = newDirectory();
@@ -184,8 +186,7 @@ public void testMultipleVectorFields() throws Exception {
         for (int field = 0; field < numVectorFields; field++) {
           float[] vector = values[field][docID];
           if (vector != null) {
-            FieldType fieldType = KnnVectorField.createFieldType(vector.length, similarityFunction);
-            doc.add(new KnnVectorField(KNN_GRAPH_FIELD + field, vector, fieldType));
+            doc.add(new KnnVectorField(KNN_GRAPH_FIELD + field, vector, fieldTypes[field]));
           }
         }
         String idString = Integer.toString(docID);
@@ -428,15 +429,18 @@ private void assertConsistentGraph(IndexWriter iw, float[][] values, String vect
     try (DirectoryReader dr = DirectoryReader.open(iw)) {
       for (LeafReaderContext ctx : dr.leaves()) {
         LeafReader reader = ctx.reader();
-        VectorValues vectorValues = reader.getVectorValues(vectorField);
         PerFieldKnnVectorsFormat.FieldsReader perFieldReader =
             (PerFieldKnnVectorsFormat.FieldsReader) ((CodecReader) reader).getVectorReader();
         if (perFieldReader == null) {
           continue;
         }
         Lucene91HnswVectorsReader vectorReader =
             (Lucene91HnswVectorsReader) perFieldReader.getFieldReader(vectorField);
+        if (vectorReader == null) {
+          continue;
+        }
         KnnGraphValues graphValues = vectorReader.getGraphValues(vectorField);
+        VectorValues vectorValues = reader.getVectorValues(vectorField);
         if (vectorValues == null) {
           assert graphValues == null;
           continue;