Override iterator in Empty off heap vector values (#13837)

This commit override the iterator method in the empty off-heap vector values. The implementation is just the dense iterator, which handles empty values just fine. We use it elsewhere for similar too.

lucene/backward-codecs/src/java/org/apache/lucene/backward_codecs/lucene92/OffHeapFloatVectorValues.java

@@ -249,6 +249,11 @@ abstract class OffHeapFloatVectorValues extends FloatVectorValues {
       throw new UnsupportedOperationException();
     }
 
+    @Override
+    public DocIndexIterator iterator() {
+      return createDenseIterator();
+    }
+
     @Override
     public Bits getAcceptOrds(Bits acceptDocs) {
       return null;

lucene/backward-codecs/src/java/org/apache/lucene/backward_codecs/lucene94/OffHeapByteVectorValues.java

@@ -263,6 +263,11 @@ abstract class OffHeapByteVectorValues extends ByteVectorValues {
       throw new UnsupportedOperationException();
     }
 
+    @Override
+    public DocIndexIterator iterator() {
+      return createDenseIterator();
+    }
+
     @Override
     public Bits getAcceptOrds(Bits acceptDocs) {
       return null;

lucene/backward-codecs/src/java/org/apache/lucene/backward_codecs/lucene94/OffHeapFloatVectorValues.java

@@ -262,6 +262,11 @@ abstract class OffHeapFloatVectorValues extends FloatVectorValues {
       throw new UnsupportedOperationException();
     }
 
+    @Override
+    public DocIndexIterator iterator() {
+      return createDenseIterator();
+    }
+
     @Override
     public Bits getAcceptOrds(Bits acceptDocs) {
       return null;