Return no match when no dimensions have been set

Author: carlosdelest

server/src/main/java/org/elasticsearch/index/mapper/vectors/DenseVectorFieldMapper.java

@@ -35,6 +35,7 @@
 import org.apache.lucene.search.FieldExistsQuery;
 import org.apache.lucene.search.KnnByteVectorQuery;
 import org.apache.lucene.search.KnnFloatVectorQuery;
+import org.apache.lucene.search.MatchNoDocsQuery;
 import org.apache.lucene.search.PatienceKnnVectorQuery;
 import org.apache.lucene.search.Query;
 import org.apache.lucene.search.join.BitSetProducer;
@@ -2530,6 +2531,9 @@ public Query createKnnQuery(
                     "to perform knn search on field [" + name() + "], its mapping must have [index] set to [true]"
                 );
             }
+            if (dims == null) {
+                return new MatchNoDocsQuery("No data has been indexed for field [" + name() + "]");
+            }
             KnnSearchStrategy knnSearchStrategy = heuristic.getKnnSearchStrategy();
             return switch (getElementType()) {
                 case BYTE -> createKnnByteQuery(