Fix postRequestWithEmptyBodyTemplateUsesMessageData missing executor mock

The test verifies ctx.transformMsg(...), which exercises the response-
processing path. Without DirectListeningExecutor, the Reactor scheduler
is unavailable and the node calls ctx.tellFailure() instead.

rule-engine/rule-engine-components/src/test/java/org/thingsboard/rule/engine/rest/TbRestApiCallNodeTest.java

@@ -556,6 +556,8 @@ public class TbRestApiCallNodeTest extends AbstractRuleNodeUpgradeTest {
         final AtomicReference<String> capturedBody = new AtomicReference<>();
         setupServerWithBodyCapture(capturedBody, latch);
 
+        given(ctx.getExternalCallExecutor()).willReturn(DirectListeningExecutor.INSTANCE);
+
         TbRestApiCallNodeConfiguration config = new TbRestApiCallNodeConfiguration().defaultConfiguration();
         config.setRequestMethod("POST");
         // requestBodyTemplate is null by default — should use msg.getData()