Label information not included, if single stack is requested.

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Describe the bug
If all stacks of a board are requested via REST API (GET /boards/{boardId}/stacks), the label information of all cards is included in the response for all stacks.

Example: This is the JSON response of a test board with only one stack and two cards, the second one containing a label.

[
   {
      "id":24,
      "title":"TestStack",
      "boardId":7,
      "deletedAt":0,
      "lastModified":1717319772,
      "cards":[
         {
            "id":833,
            "title":"TestCard1",
            "description":"",
            "stackId":24,
            "type":"plain",
            "lastModified":1717319754,
            "lastEditor":null,
            "createdAt":1717194622,
            "labels":[
               
            ],
            "assignedUsers":[
               
            ],
            "attachments":null,
            "attachmentCount":0,
            "owner":{
               "primaryKey":"Tester",
               "uid":"Tester",
               "displayname":"Max Mustermann",
               "type":0
            },
            "order":999,
            "archived":false,
            "done":null,
            "duedate":null,
            "deletedAt":0,
            "commentsUnread":0,
            "commentsCount":0,
            "ETag":"2ce44df456088877165c8ffb445da29a",
            "overdue":0
         },
         {
            "id":834,
            "title":"TestCard2",
            "description":"",
            "stackId":24,
            "type":"plain",
            "lastModified":1717319772,
            "lastEditor":null,
            "createdAt":1717194626,
            "labels":[
               {
                  "id":187,
                  "title":"TestTag",
                  "color":"c4be27",
                  "boardId":7,
                  "cardId":834,
                  "lastModified":0,
                  "ETag":"cfcd208495d565ef66e7dff9f98764da"
               }
            ],
            "assignedUsers":[
               
            ],
            "attachments":null,
            "attachmentCount":0,
            "owner":{
               "primaryKey":"Tester",
               "uid":"Tester",
               "displayname":"Max Mustermann",
               "type":0
            },
            "order":999,
            "archived":false,
            "done":null,
            "duedate":null,
            "deletedAt":0,
            "commentsUnread":0,
            "commentsCount":0,
            "ETag":"203884e255b00b4169807fb0baa3837f",
            "overdue":0
         }
      ],
      "order":999,
      "ETag":"203884e255b00b4169807fb0baa3837f"
   }
]

But if information of an individual stack of a board is requested via REST API (GET /boards/{boardId}/stacks/{stackId}), the label information of the cards is missing in the JSON response. The field of the labels is "null.

This is the JSON response for the same stack, requested for the stack individually:

{
   "id":24,
   "title":"TestStack",
   "boardId":7,
   "deletedAt":0,
   "lastModified":1717319772,
   "cards":[
      {
         "id":833,
         "title":"TestCard1",
         "description":"",
         "stackId":24,
         "type":"plain",
         "lastModified":1717319754,
         "lastEditor":null,
         "createdAt":1717194622,
         "labels":null,
         "assignedUsers":[
            
         ],
         "attachments":null,
         "attachmentCount":0,
         "owner":"Tester",
         "order":999,
         "archived":false,
         "done":null,
         "duedate":null,
         "deletedAt":0,
         "commentsUnread":0,
         "commentsCount":0,
         "ETag":"2ce44df456088877165c8ffb445da29a",
         "overdue":0
      },
      {
         "id":834,
         "title":"TestCard2",
         "description":"",
         "stackId":24,
         "type":"plain",
         "lastModified":1717319772,
         "lastEditor":null,
         "createdAt":1717194626,
         "labels":null,
         "assignedUsers":[
            
         ],
         "attachments":null,
         "attachmentCount":0,
         "owner":"Tester",
         "order":999,
         "archived":false,
         "done":null,
         "duedate":null,
         "deletedAt":0,
         "commentsUnread":0,
         "commentsCount":0,
         "ETag":"203884e255b00b4169807fb0baa3837f",
         "overdue":0
      }
   ],
   "order":999,
   "ETag":"203884e255b00b4169807fb0baa3837f"
}

To Reproduce
Steps to reproduce the behavior:

1. Set up a Nextcloud Deck board with a stack containing a card with a label
2. curl -X GET 'https://<username>:<password>@<NextcloudIPorURL>/index.php/apps/deck/api/v1.1/boards/' -H 'Accept: application/json' -H 'OCS-APIRequest: true' -H 'Content-Type: application/json;charset=utf-8'
3. Check, if label is included in the response
4. curl -X GET 'https://<username>:<password>@<NextcloudIPorURL>/index.php/apps/deck/api/v1.1/boards/<boardID>/stacks/<stackID>' -H 'Accept: application/json' -H 'OCS-APIRequest: true' -H 'Content-Type: application/json;charset=utf-8'
5. Check, if label is not included in the response.

Expected behavior
In case the data of an individual stack is requested, the label information should be included in the JSON resonse.

Client details:

• OS: iOS and macOS
• Version: iOS 17, macOS 14
• Device: iPhone Xs, iPhone 15 Simulator, macBook Air M1

Start by comparing the GET /boards/{boardId}/stacks and GET /boards/{boardId}/stacks/{stackId} REST responses using the reproduction curl commands. Trace the individual-stack endpoint and its card serialization, then verify that cards returned by it include the same label data as the all-stacks response.