Initialize severity value with 0 in powerfilter (backport #3031)

[mergify]

This is an automatic backport of pull request #3031 done by Mergify.
Cherry-pick of 4276eaa has failed:

On branch mergify/bp/gsa-21.04/pr-3031
Your branch is ahead of 'origin/gsa-21.04' by 1 commit.
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You are currently cherry-picking commit 4276eaad2.
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Unmerged paths:
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	both modified:   CHANGELOG.md

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

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[codecov]

Codecov Report

Merging #3032 (e964a28) into gsa-21.04 (93fc08d) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           gsa-21.04    #3032   +/-   ##
==========================================
  Coverage      54.29%   54.29%           
==========================================
  Files           1076     1076           
  Lines          26245    26245           
  Branches        7490     7490           
==========================================
  Hits           14249    14249           
  Misses         10879    10879           
  Partials        1117     1117           

Impacted Files	Coverage Δ
gsa/src/web/pages/results/filterdialog.js	37.50% <ø> (ø)
.../web/components/powerfilter/severityvaluesgroup.js	91.66% <100.00%> (ø)

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