User Contribution Mapping - Improve Download CSV UI for when there are no placeholders
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### 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
{width=981 height=217}
2. the modal opens showing the **_Download with prefilled CSV template_** button
{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
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