Correctly handle scenario when no top-level groups are available

Problem to solve

When a user is unable to create a top-level group (due to permissions) and no top-level groups are available for that user to migrate into, the UI does not handle this scenario. UI blows up (due to error in console) and displays empty dropdown instead of "No parent" being preselected

Proposed solution

Show a simple message to the user: You must have at least one top-level group with write permission before you could perform an import.

Select "No parent" as default for this case and do not blow up due to missing groups. This functionality depends on !54323 (merged)

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