Fixes #6344: Adds null check for adjust campaign

[sblatz]

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• Quality: This PR builds and passes detekt/ktlint checks (A pre-push hook is recommended)
• Tests: This PR includes thorough tests or an explanation of why it does not
• Screenshots: This PR includes screenshots or GIFs of the changes made or an explanation of why it does not
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[codecov-io]

Codecov Report

Merging #6348 into master will decrease coverage by <.01%.
The diff coverage is 0%.

@@             Coverage Diff              @@
##             master    #6348      +/-   ##
============================================
- Coverage     14.47%   14.47%   -0.01%     
  Complexity      323      323              
============================================
  Files           272      272              
  Lines         11037    11038       +1     
  Branches       1593     1594       +1     
============================================
  Hits           1598     1598              
- Misses         9310     9311       +1     
  Partials        129      129

Impacted Files	Coverage Δ	Complexity Δ
...a/fenix/components/metrics/AdjustMetricsService.kt	3.44% <0%> (-0.13%)	1 <0> (ø)

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[csadilek]

🚢

Since this is easy to break and causing lots of crashes when it does, let's file a follow-up to guard with a test? This will likely take a bit of time, so let's land to fix the crashes.