Only show empty states if loaded

[jacbn]

Set Assignments: on Ada particularly, a big yellow warning appears for a fraction of a scond while loading Set Assignments currently, as the empty state box appears while the query is loading. While I would like to replace this entire file with a <ShowLoadingQuery/> where we can make use of placeholders, since we have this as is it is much easier to simply adjust the logic not to show these empty states if groups/boards are undefined.

User search: "no results" particularly noticeable during postcode search on a large group. Now shows the <Loading/> component.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 41.68%. Comparing base (c5c833e) to head (2ef80c1).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1831      +/-   ##
==========================================
- Coverage   41.73%   41.68%   -0.05%     
==========================================
  Files         542      542              
  Lines       23685    23686       +1     
  Branches     7855     7856       +1     
==========================================
- Hits         9884     9874      -10     
- Misses      13757    13768      +11     
  Partials       44       44              

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