NETOBSERV-1090: initialize empty slice for returning json [1.3 backport]

[jotak]

backport of #340

[openshift-ci-robot]

@jotak: This pull request references NETOBSERV-1090 which is a valid jira issue.

In response to this:

backport of #340

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[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (release-1.3@7036d27). Click here to learn what that means.
The diff coverage is n/a.

@@              Coverage Diff               @@
##             release-1.3     #341   +/-   ##
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  Coverage               ?   57.83%           
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  Files                  ?      150           
  Lines                  ?     6653           
  Branches               ?      795           
==============================================
  Hits                   ?     3848           
  Misses                 ?     2588           
  Partials               ?      217           

Flag	Coverage Δ
uitests	58.77% <0.00%> (?)
unittests	55.21% <0.00%> (?)

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[jpinsonneau]

/lgtm

[Amoghrd]

/ok-to-test

[github-actions]

New image: quay.io/netobserv/network-observability-console-plugin:78c25b9. It will expire after two weeks.

[Amoghrd]

/label qe-approved

[jotak]

/approve

[openshift-ci]

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