fix: check for nil client before closing in amqp (#10635)

(cherry picked from commit 0c4df85)

plugins/outputs/amqp/amqp.go

@@ -259,7 +259,7 @@ func (q *AMQP) Write(metrics []telegraf.Metric) error {
 				if err != nil {
 					return err
 				}
-			} else {
+			} else if q.client != nil {
 				if err := q.client.Close(); err != nil {
 					q.Log.Errorf("Closing connection failed: %v", err)
 				}