Fix listener leak in TransportExecuteMonitorAction for empty source (#2080)

Add else block when getResponse.isSourceEmpty is true to call
actionListener.onFailure() instead of silently dropping the request.
Without this, the listener is never notified and the thread hangs.

Signed-off-by: Manaswini Ragamouni <ragamanu@amazon.com>
Co-authored-by: Manaswini Ragamouni <ragamanu@amazon.com>

Author: manaswini1920

alerting/src/main/kotlin/org/opensearch/alerting/transport/TransportExecuteMonitorAction.kt

@@ -135,6 +135,15 @@ class TransportExecuteMonitorAction @Inject constructor(
                                 val monitor = ScheduledJob.parse(xcp, getResponse.id, getResponse.version) as Monitor
                                 executeMonitor(monitor)
                             }
+                        } else {
+                            actionListener.onFailure(
+                                AlertingException.wrap(
+                                    OpenSearchStatusException(
+                                        "Monitor source is empty for id: ${execMonitorRequest.monitorId}",
+                                        RestStatus.NOT_FOUND
+                                    )
+                                )
+                            )
                         }
                     } catch (e: Exception) {
                         log.error("Failed to get monitor ${execMonitorRequest.monitorId} for execution", e)