Resolve "Fix nil paths in group migration history"

What does this MR do and why?

This MR replaces nil in group migration history with loading icons when target path is not yet known

Screenshots or screen recordings

How to set up and validate locally

• Enable feature flag :bulk_import_projects in rails console: Feature.enable(:bulk_import_projects)
• In GitLab admin settings turn on Enable migrating GitLab groups and projects by direct transfer and save changes /admin/application_settings/general#js-visibility-settings
• Open "New group" -> Import (/groups/new)
• Use https://gitlab.com as source instance and GeK1Nis4j-SY1X4sqE5c as personal access token (this token is from separate user on GitLab instance with 0 real data available, so we do not expose any security risks here)
• Start importing top-level-group-1 (it contains projects) with projects
• Immediately open History page (on top) and start refreshing (there are no realtime updates yet) to see "loading icons"

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #388193 (closed)