core: add remaining nil checks

[dB2510]

Add remaining nil checks that were missing as part of OBOL-03.

category: misc
ticket: #1997

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: +0.08 🎉

Comparison is base (db040d5) 53.56% compared to head (09625ef) 53.65%.

❗ Current head 09625ef differs from pull request most recent head 96fd018. Consider uploading reports for the commit 96fd018 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2338      +/-   ##
==========================================
+ Coverage   53.56%   53.65%   +0.08%     
==========================================
  Files         191      191              
  Lines       25730    25735       +5     
==========================================
+ Hits        13783    13808      +25     
+ Misses      10238    10219      -19     
+ Partials     1709     1708       -1     

Impacted Files	Coverage Δ
core/consensus/msg.go	73.33% <0.00%> (-0.40%)	⬇️
core/priority/prioritiser.go	62.32% <0.00%> (+1.94%)	⬆️

... and 11 files with indirect coverage changes

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