[APPEND][FSTORE-1135] Fix preview embedding in the UI when pk is not …

…in the opensearch schema (#1765)

(cherry picked from commit af7a7dec5a7c084f264c03364e70416b32c815e5)

vector-db/src/main/java/io/hops/hopsworks/vectordb/OpensearchVectorDatabase.java

@@ -203,6 +203,12 @@ public void batchWrite(Index index, Map<String, String> data) throws VectorDatab
  public List<Map<String, Object>> preview(Index index, Set<Field> fields, int n) throws VectorDatabaseException {
  List<Map<String, Object>> results = Lists.newArrayList();
 
+ // If fields is empty, return no result, otherwise the search query matches any documents which may not
+ // belong to requested feature group.
+ if (fields.size() == 0) {
+ return results;
+ }
+
  // Create a bool query
  BoolQueryBuilder boolQueryBuilder = QueryBuilders.boolQuery();