MINIFICPP-2498 Funnels should be usable as terminators

The output relationship of a funnel is now auto-terminated by default.
You can connect it to a processor's input, but you don't have to.

docker/test/integration/features/core_functionality.feature

@@ -39,6 +39,14 @@ Feature: Core flow functionalities
     Then at least one flowfile with the content "first_custom_text" is placed in the monitored directory in less than 20 seconds
     And at least one flowfile with the content "second_custom_text" is placed in the monitored directory in less than 20 seconds
 
+  @CORE
+  Scenario: A funnel can be used as a terminator
+    Given a GenerateFlowFile processor with the "Data Format" property set to "Text"
+    And a Funnel with the name "TerminalFunnel" is set up
+    And the "success" relationship of the GenerateFlowFile processor is connected to the TerminalFunnel
+    When the MiNiFi instance starts up
+    Then the Minifi logs do not contain the following message: "Connect empty for non auto terminated relationship" after 5 seconds
+
   @CORE
   Scenario: The default configuration uses RocksDB for both the flow file and content repositories
     Given a GenerateFlowFile processor with the "Data Format" property set to "Text"

libminifi/include/Funnel.h

@@ -31,6 +31,8 @@ class Funnel final : public ForwardingNode {
 
   MINIFIAPI static constexpr core::annotation::Input InputRequirement = core::annotation::Input::INPUT_REQUIRED;
   ADD_COMMON_VIRTUAL_FUNCTIONS_FOR_PROCESSORS
+
+  void onSchedule(core::ProcessContext&, core::ProcessSessionFactory&) override;
 };
 
 }  // namespace org::apache::nifi::minifi

libminifi/src/Funnel.cpp

@@ -0,0 +1,26 @@
+/**
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "Funnel.h"
+
+namespace org::apache::nifi::minifi {
+
+void Funnel::onSchedule(core::ProcessContext&, core::ProcessSessionFactory&) {
+  addAutoTerminatedRelationship(Success);
+}
+
+}  // namespace org::apache::nifi::minifi