[JENKINS-59178] Fix NPE when aggregating different results in matrix build

[uhafner]

See JENKINS-59178.

[codecov]

Codecov Report

Merging #195 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##             master    #195      +/-   ##
===========================================
- Coverage     82.81%   82.8%   -0.01%     
  Complexity     1542    1542              
===========================================
  Files           245     245              
  Lines          5296    5293       -3     
  Branches        376     375       -1     
===========================================
- Hits           4386    4383       -3     
  Misses          768     768              
  Partials        142     142

Impacted Files	Coverage Δ	Complexity Δ
.../plugins/analysis/core/steps/IssuesAggregator.java	100% <100%> (ø)	8 <5> (-2)	⬇️
...s/plugins/analysis/core/steps/AnnotatedReport.java	100% <100%> (ø)	19 <4> (+2)	⬆️

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