Account for 0 graph size when initializing HNSW graph (apache#13964)

When initializing a joint graph from one of the segments' graphs,
 we always assume that a segment's graph is present. But later we want
 to explore an option where some segments will not have graphs (apache#13447).

This change allows to account for missing graphs.

lucene/core/src/java/org/apache/lucene/util/hnsw/ConcurrentHnswMerger.java

@@ -48,28 +48,23 @@ public ConcurrentHnswMerger(
   @Override
   protected HnswBuilder createBuilder(KnnVectorValues mergedVectorValues, int maxOrd)
       throws IOException {
+    OnHeapHnswGraph graph;
+    BitSet initializedNodes = null;
+
     if (initReader == null) {
-      return new HnswConcurrentMergeBuilder(
-          taskExecutor,
-          numWorker,
-          scorerSupplier,
-          M,
-          beamWidth,
-          new OnHeapHnswGraph(M, maxOrd),
-          null);
+      graph = new OnHeapHnswGraph(M, maxOrd);
+    } else {
+      HnswGraph initializerGraph = ((HnswGraphProvider) initReader).getGraph(fieldInfo.name);
+      if (initializerGraph.size() == 0) {
+        graph = new OnHeapHnswGraph(M, maxOrd);
+      } else {
+        initializedNodes = new FixedBitSet(maxOrd);
+        int[] oldToNewOrdinalMap = getNewOrdMapping(mergedVectorValues, initializedNodes);
+        graph =
+            InitializedHnswGraphBuilder.initGraph(M, initializerGraph, oldToNewOrdinalMap, maxOrd);
+      }
     }
-
-    HnswGraph initializerGraph = ((HnswGraphProvider) initReader).getGraph(fieldInfo.name);
-    BitSet initializedNodes = new FixedBitSet(maxOrd);
-    int[] oldToNewOrdinalMap = getNewOrdMapping(mergedVectorValues, initializedNodes);
-
     return new HnswConcurrentMergeBuilder(
-        taskExecutor,
-        numWorker,
-        scorerSupplier,
-        M,
-        beamWidth,
-        InitializedHnswGraphBuilder.initGraph(M, initializerGraph, oldToNewOrdinalMap, maxOrd),
-        initializedNodes);
+        taskExecutor, numWorker, scorerSupplier, M, beamWidth, graph, initializedNodes);
   }
 }

lucene/core/src/java/org/apache/lucene/util/hnsw/IncrementalHnswGraphMerger.java

@@ -121,6 +121,10 @@ protected HnswBuilder createBuilder(KnnVectorValues mergedVectorValues, int maxO
     }
 
     HnswGraph initializerGraph = ((HnswGraphProvider) initReader).getGraph(fieldInfo.name);
+    if (initializerGraph.size() == 0) {
+      return HnswGraphBuilder.create(
+          scorerSupplier, M, beamWidth, HnswGraphBuilder.randSeed, maxOrd);
+    }
 
     BitSet initializedNodes = new FixedBitSet(maxOrd);
     int[] oldToNewOrdinalMap = getNewOrdMapping(mergedVectorValues, initializedNodes);