Fix loading dense_vectors when no data has been indexed (no dims specified)

Author: carlosdelest

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

@@ -2577,6 +2577,11 @@ public BlockLoader blockLoader(MappedFieldType.BlockLoaderContext blContext) {
                 return null;
             }
 
+            if (dims == null) {
+                // No data has been indexed yet
+                return BlockLoader.CONSTANT_NULLS;
+            }
+
             if (indexed) {
                 return new BlockDocValuesReader.DenseVectorBlockLoader(name(), dims);
             }

x-pack/plugin/esql/qa/server/src/main/java/org/elasticsearch/xpack/esql/qa/rest/RestEsqlTestCase.java

@@ -1022,7 +1022,7 @@ public void testMultipleBatchesWithLookupJoin() throws IOException {
             var query = requestObjectBuilder().query(format(null, "from * | lookup join {} on integer {}", testIndexName(), sort));
             Map<String, Object> result = runEsql(query);
             var columns = as(result.get("columns"), List.class);
-            assertEquals(21, columns.size());
+            assertEquals(22, columns.size());
             var values = as(result.get("values"), List.class);
             assertEquals(10, values.size());
         }

x-pack/plugin/esql/qa/testFixtures/src/main/resources/mapping-all-types.json

@@ -63,6 +63,9 @@
         "semantic_text": {
             "type": "semantic_text",
             "inference_id": "foo_inference_id"
+        },
+        "dense_vector": {
+            "type": "dense_vector"
         }
     }
 }