core: add nil checks

[dB2510]

Adds nil checks before reading proto messages in core package mentioned in audit report.

Refer: OBOL-03

category: misc
ticket: #1997

[codecov]

Codecov Report

Patch coverage: 50.00% and project coverage change: -0.05 ⚠️

Comparison is base (0dd410d) 53.63% compared to head (620bf60) 53.58%.

❗ Current head 620bf60 differs from pull request most recent head 2fca490. Consider uploading reports for the commit 2fca490 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2090      +/-   ##
==========================================
- Coverage   53.63%   53.58%   -0.05%     
==========================================
  Files         174      174              
  Lines       22671    22715      +44     
==========================================
+ Hits        12159    12172      +13     
- Misses       9004     9026      +22     
- Partials     1508     1517       +9     

Impacted Files	Coverage Δ
core/parsigex/parsigex.go	62.79% <0.00%> (-2.27%)	⬇️
core/priority/component.go	0.00% <0.00%> (ø)
core/proto.go	64.10% <100.00%> (+2.19%)	⬆️

... and 7 files with indirect coverage changes

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[dB2510]

@corverroos
A short note: The audit report also mentioned to add check in core/proto.go::DutyFromProto(). I didn't added that as we already do nil checks before passing duty proto to this function wherever it is called. Also adding check to this will modify function signature to return error which impacts uses of this function some other places as well. Not worth it IMO. WDYT?