add empty result set when single count query

include/caosdb/transaction.h

@@ -389,6 +389,23 @@ public:
     return this->request->requests_size();
   }
 
+  /**
+   * Get a JSON representation of the respone.
+   *
+   * For debugging.
+   */
+  inline auto ResponseToString() const -> const std::string {
+    google::protobuf::util::JsonOptions options;
+    std::string out;
+    google::protobuf::util::MessageToJsonString(*this->response, &out, options);
+    return out;
+  }
+
+  /**
+   * Get a JSON representation of the request.
+   *
+   * For debugging.
+   */
   inline auto RequestToString() const -> const std::string {
     google::protobuf::util::JsonOptions options;
     std::string out;

src/caosdb/transaction.cpp

@@ -300,6 +300,9 @@ auto Transaction::WaitForIt() const noexcept -> TransactionStatus {
       } break;
       case QueryResponseCase::kCountResult: {
         this->query_count = retrieve_response->count_result();
+        std::vector<std::unique_ptr<Entity>> entities;
+        this->result_set =
+          std::make_unique<MultiResultSet>(std::move(entities));
       } break;
       default:
         // TODO(tf) Error