docker_swarm_service: ensure idempotency when the user parameter is None (#49235)

* ensure idempotency for user set to None

* Update `user` documentation and add changelog fragment

* clarify changelog fragments and parameters documentation

* use restructuredtext syntax in changelog fragment

changelogs/fragments/49235-docker_swarm_service-user-default.yaml

@@ -0,0 +1,5 @@
+---
+bugfixes:
+  - 'docker_swarm_service: fails because of default "user: root" (https://github.com/ansible/ansible/issues/49199)'
+minor_changes:
+  - 'docker_swarm_service: use docker defaults for the ``user`` parameter if it is set to ``null``'

lib/ansible/modules/cloud/docker/docker_swarm_service.py

@@ -271,7 +271,10 @@ options:
   user:
     required: false
     default: root
-    description: username or UID
+    description:
+    - username or UID.
+    - "If set to C(null) the image provided value (or the one already
+       set for the service) will be used"
 extends_documentation_fragment:
 - docker
 requirements:
@@ -720,7 +723,7 @@ class DockerService(DockerBaseClass):
             differences.add('update_order', parameter=self.update_order, active=os.update_order)
         if self.image != os.image.split('@')[0]:
             differences.add('image', parameter=self.image, active=os.image.split('@')[0])
-        if self.user != os.user:
+        if self.user and self.user != os.user:
             differences.add('user', parameter=self.user, active=os.user)
         if self.dns != os.dns:
             differences.add('dns', parameter=self.dns, active=os.dns)