app/protonil: implement generic nil proto checker

[corverroos]

Implements a generic protobuf nil checker to mitigate against manual verbose or buggy protobuf nil checks.

Note this requires fields that allowed to be nil to be specified as optional in the proto definition.

category: misc
ticket: none

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.10 🎉

Comparison is base (db040d5) 53.56% compared to head (f4b297b) 53.67%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2344      +/-   ##
==========================================
+ Coverage   53.56%   53.67%   +0.10%     
==========================================
  Files         191      192       +1     
  Lines       25730    25775      +45     
==========================================
+ Hits        13783    13834      +51     
+ Misses      10238    10233       -5     
+ Partials     1709     1708       -1     

Impacted Files	Coverage Δ
app/protonil/protonil.go	100.00% <100.00%> (ø)

... and 9 files with indirect coverage changes

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