**Apache Airflow version**: 2.1.0

**What happened**:

Using a Label infront of a TaskGroup breaks task dependencies, as tasks before the TaskGroup will now point to the last task in the TaskGroup.

**What you expected to happen**:

Task dependencies with or without Labels should be the same (and preferably the Label should be visible outside of the TaskGroup).
Behavior without a Label is as follows:

```python
with TaskGroup("tg1", tooltip="Tasks related to task group") as tg1:
        DummyOperator(task_id="b") >> DummyOperator(task_id="c")

DummyOperator(task_id="a") >> tg1
```
![without Taskgroup](https://user-images.githubusercontent.com/35238779/128502690-58b31577-91d7-4b5c-b348-4779c9286958.png)

**How to reproduce it**:

```python
with TaskGroup("tg1", tooltip="Tasks related to task group") as tg1:
        DummyOperator(task_id="b") >> DummyOperator(task_id="c")

    DummyOperator(task_id="a") >> Label("testlabel") >> tg1
```
![with TaskGroup](https://user-images.githubusercontent.com/35238779/128502758-5252b7e7-7c30-424a-ac3a-3f9a99b0a805.png)


}
        )
    if task_group.downstream_group_ids or task_group.downstream_task_ids:
        
        children.append(
            {
                "id": task_group.downstream_join_id,
                "value": {
                    "label": "",
                    "labelStyle": f"fill:{task_group.ui_fgcolor};",
                    "style": f"fill:{task_group.ui_color};",
                    "shape": "circle",
                },
            }
        )
    return {
        "id": task_group.group_id,
        "value": {
            "label": task_group.label,
            "labelStyle": f"fill:{task_group.ui_fgcolor};",
            "style": f"fill:{task_group.ui_color}",
            "rx": 5,
            "ry": 5,
            "clusterLabelPos": "top",
            "tooltip": task_group.tooltip,
            "isMapped": is_mapped,
        },
        "children": children,
    }