Use UNSET_VALUE as default for relation of severity values in powerfilter (backport #3052)

[mergify]

This is an automatic backport of pull request #3052 done by Mergify.

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[codecov]

Codecov Report

Merging #3054 (84947f9) into gsa-21.10 (d319838) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           gsa-21.10    #3054   +/-   ##
==========================================
  Coverage      56.39%   56.39%           
==========================================
  Files           1036     1036           
  Lines          25183    25183           
  Branches        7199     7199           
==========================================
  Hits           14203    14203           
  Misses          9923     9923           
  Partials        1057     1057           

Impacted Files	Coverage Δ
...src/web/components/powerfilter/relationselector.js	100.00% <100.00%> (ø)
.../web/components/powerfilter/severityvaluesgroup.js	91.66% <100.00%> (ø)

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