fix(frontend): set defaults for data-fetching hooks

[eatyourgreens]

Return empty arrays by default from useUnits and useVariables.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.55%. Comparing base (c227f57) to head (e4e5929).
Report is 28 commits behind head on ui-enhancements.

Additional details and impacted files

@@                 Coverage Diff                 @@
##           ui-enhancements     #591      +/-   ##
===================================================
+ Coverage            76.17%   76.55%   +0.37%     
===================================================
  Files                  108      111       +3     
  Lines                 5600     5651      +51     
===================================================
+ Hits                  4266     4326      +60     
+ Misses                1334     1325       -9     

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[sonarqubecloud]

Quality Gate passed for 'pkpdapp'

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
91.7% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[sonarqubecloud]

Quality Gate failed for 'pkpdapp-team_pkpdapp_frontend'

Failed conditions
0.0% Coverage on New Code (required ≥ 80%)
C Reliability Rating on New Code (required ≥ A)

See analysis details on SonarQube Cloud

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