Normalize layers/groups management in TOC

[tdipisa]

Description

To maintain retro-compatibility with old maps, as reported here in https://github.com/geosolutions-it/MapStore2/issues/10246, with the new TOC there are still a few inconsistent but not blocking behaviors due to how the 'default group' is managed.

Loading a Map in one of the following scenarios:

• Importing a Map
• Opening an existing Map

a logic should intervene to automatically update the TOC internal structure to definitely fix retro-compatibility issues.

What kind of improvement you want to add? (check one with "x", remove the others)

• [X] Minor changes to existing features
• [ ] Code style update (formatting, local variables)
• [ ] Refactoring (no functional changes, no api changes)
• [ ] Build related changes
• [ ] CI related changes
• [ ] Other... Please describe:

Other useful information

[mahmoudadel54]

What I understood from the description, for some old maps, it is requested to handle the logic of adding layer without group in case not selecting any existing group on TOC before adding. is it correct ?

The current implemented code is Ok with any new created map based on this PR: https://github.com/geosolutions-it/MapStore2/pull/10278, right ?