Handle properly when trying to serialize a zero documents cluster in IVF (elastic#129559)

We currently fail to serialize this case. This change has been made so
it is backwards compatible. I label it as non an issue as this feature
is behind a feature flag and is not released.

fixes elastic#129551

Author: iverase

server/src/main/java/org/elasticsearch/index/codec/vectors/DocIdsWriter.java

@@ -70,15 +70,14 @@ final class DocIdsWriter {
     DocIdsWriter() {}
 
     void writeDocIds(IntToIntFunction docIds, int count, DataOutput out) throws IOException {
-        // docs can be sorted either when all docs in a block have the same value
-        // or when a segment is sorted
         if (count == 0) {
-            out.writeByte(CONTINUOUS_IDS);
             return;
         }
         if (count > scratch.length) {
             scratch = new int[count];
         }
+        // docs can be sorted either when all docs in a block have the same value
+        // or when a segment is sorted
         boolean strictlySorted = true;
         int min = docIds.apply(0);
         int max = min;
@@ -215,6 +214,9 @@ private static void writeIdsAsBitSet(IntToIntFunction docIds, int count, DataOut
 
     /** Read {@code count} integers into {@code docIDs}. */
     void readInts(IndexInput in, int count, int[] docIDs) throws IOException {
+        if (count == 0) {
+            return;
+        }
         if (count > scratch.length) {
             scratch = new int[count];
         }

server/src/test/java/org/elasticsearch/index/codec/vectors/DocIdsWriterTests.java

@@ -40,6 +40,12 @@
 
 public class DocIdsWriterTests extends LuceneTestCase {
 
+    public void testNoDocs() throws Exception {
+        try (Directory dir = newDirectory()) {
+            test(dir, new int[0]);
+        }
+    }
+
     public void testRandom() throws Exception {
         int numIters = atLeast(100);
         try (Directory dir = newDirectory()) {