Fix loading of relational fields in group

[Nitwel]

Description

The problem was that when opening the group it didn't load the children for the children in the group, thus displaying no arrows inside a group even if there are relational fields. The reason why no inner children got loaded was caused by the group not having any value which is intended as groups are alias fields.

The fix was to preload any children of children of groups and if we open a child in a group, to be able to find that child as the path is not matching the actual tree, and then to load it's children.

Here is the problem visualized:

- "field1"
- "field2"
- "" <--- Is a group but has no path as "relational-field-in-group" is top-level for the api
  - "relational-field-in-group" <--- by giving the group no path it's chidren are correct
- "field4"

When you now click on that group we would have to somehow find it in the tree with the path of "" which we can't as the path is nonsense.

The solution is to do this:

- "field1"
- "field2"
- ""
  - "relational-field-in-group" <-- we are now able to find this even though the path would in theory be "group1.relational-field-in-group".
    - "relational-field-in-group.relational-field1"
    - "relational-field-in-group.relational-field2"
- "field4"

Fixes #13899

[paescuj]

Fix makes sense to me 👍