User Contribution Mapping - Improve Download CSV UI for when there are no placeholders
<!--IssueSummary start--> <details> <summary> Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards. </summary> - [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=516228) </details> <!--IssueSummary end--> ### Problem Follows up on https://gitlab.com/gitlab-org/gitlab/-/issues/512590#note_2313362261. We want to improve the user experience when there are no placeholders left to reassign. Currently the flow is: 1. the user clicks the **_Reassign with CSV file_** button ![Screenshot_2025-01-22_at_6.24.50_PM.png](/uploads/2c1d38d794780edf60c2975028913e2a/Screenshot_2025-01-22_at_6.24.50_PM.png){width=981 height=217} 2. the modal opens showing the **_Download with prefilled CSV template_** button ![Screenshot_2025-01-22_at_6.21.19_PM.png](/uploads/ee0f2316e30d4a0f6804194809e770d6/Screenshot_2025-01-22_at_6.21.19_PM.png){width=935 height=493} 3. only when the user clicks on the button are they informed that there are no users to reassign ### Solution The Reassign with CSV button is hidden (or disabled?) if the CSV template is empty. The availability of placeholder users to reassign (and/or the data in the CSV) would therefore need to be surfaced on page load when navigating to groups/\<GROUPNAME\>/-/group_members?tab=placeholders&subtab=awaiting <!--template sourced from https://gitlab.com/gitlab-org/gitlab/-/blob/master/.gitlab/issue_templates/Default.md-->
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