Remove default group_label substitution with "Variable"

[SultanOrazbayev]

As discussed here: #430

[maximlt]

Unfortunately, it looks like it's not that easy to just not set group_label, it's used in various places throughout the code and is expected not to be None. @SultanOrazbayev if you plan to continue working on this PR (that may be a lot more work!), I think a good starting point would be to list all the cases where group_label is relied upon, with snippets and screenshots.

[SultanOrazbayev]

@maximlt : Thank you for the comments and pointers! Yes, I realized that the fix is going to be a bit more complex. I'll try to resolve this, adding relevant info (screenshots, etc) to the original issue.

I'm not sure about the timeline, though, is there a preference in terms of draft PRs for this repo? (I put it as a draft PR as it's easier for me to track my own PRs to work on).

[maximlt]

I'm not sure about the timeline, though, is there a preference in terms of draft PRs for this repo?

Your branch as Draft is fine to me! You can leave it open as long as you want.