fix: Prevent possible nil-pointer deref in normalizer

[jannfis]

Fix a possible nil-pointer deref in diff normalizer.

Ref: https://oss-fuzz.com/testcase-detail/6263347908050944

/cc @terrytangyuan @hblixt @AdamKorcz

Signed-off-by: jannfis jann@mistrust.net

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[AdamKorcz]

cc @DavidKorczynski

[terrytangyuan]

LGTM

[codecov]

Codecov Report

Merging #8185 (babe802) into master (cb47740) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #8185   +/-   ##
=======================================
  Coverage   41.53%   41.53%           
=======================================
  Files         174      174           
  Lines       22705    22707    +2     
=======================================
+ Hits         9430     9432    +2     
  Misses      11921    11921           
  Partials     1354     1354           

Impacted Files	Coverage Δ
util/argo/normalizers/diff_normalizer.go	77.01% <100.00%> (+0.54%)	⬆️
util/settings/settings.go	47.01% <0.00%> (ø)

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