fix: show - for empty data in searches table [ET-749]

[EmilyBonar]

Ticket

ET-749

Description

Test Plan

Checklist

• Changes have been manually QA'd
• New features have been approved by the corresponding PM
• User-facing API changes have the "User-facing API Change" label
• Release notes have been added as a separate file under docs/release-notes/
  See Release Note for details.
• Licenses have been included for new code which was copied and/or modified from any external code

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[codecov]

Codecov Report

Attention: Patch coverage is 18.34862% with 89 lines in your changes missing coverage. Please review.

Project coverage is 50.44%. Comparing base (0db2e3b) to head (01d87f6).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
webui/react/src/components/Searches/columns.ts	15.38%	88 Missing ⚠️
webui/react/src/pages/FlatRuns/columns.ts	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9963      +/-   ##
==========================================
- Coverage   54.46%   50.44%   -4.02%     
==========================================
  Files        1252      943     -309     
  Lines      156703   128119   -28584     
  Branches     3600     3600              
==========================================
- Hits        85341    64625   -20716     
+ Misses      71229    63361    -7868     
  Partials      133      133              

Flag	Coverage Δ
harness	?
web	54.28% <18.34%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
.../react/src/components/ExperimentActionDropdown.tsx	79.79% <100.00%> (ø)
webui/react/src/pages/FlatRuns/columns.ts	18.98% <75.00%> (ø)
webui/react/src/components/Searches/columns.ts	50.80% <15.38%> (+0.39%)	⬆️

... and 309 files with indirect coverage changes