fix(search): prevent facet requests from firing with empty query

[thostetler]

Facet API requests were sometimes sent with an empty q parameter,
producing 4xx errors. This was caused by a race condition where
useGetFacetData read latestQuery from the Zustand store before
submitQuery() updated it after the main search response arrived.
latestQuery defaults to q: '', so facets fired requests before the
real query was in place.

• Added empty-query guard on the enabled flag in useGetFacetData, matching the existing pattern in YearHistogramSlider and OverviewPageContainer
• Added tests for empty, whitespace-only, and valid query cases

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.8%. Comparing base (968d8a6) to head (c22a2b4).

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #801     +/-   ##
========================================
+ Coverage    61.6%   61.8%   +0.3%     
========================================
  Files         312     313      +1     
  Lines       36000   36230    +230     
  Branches     1594    1622     +28     
========================================
+ Hits        22148   22375    +227     
- Misses      13815   13818      +3     
  Partials       37      37             

Files with missing lines	Coverage Δ
src/components/SearchFacet/useGetFacetData.ts	77.9% <100.0%> (ø)

... and 5 files with indirect coverage changes

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