go/extra/stats: handle nil-votes and non-registered nodes

[ptrus]

• tested it on current state of the questnet

[codecov]

Codecov Report

Merging #2566 into master will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2566      +/-   ##
==========================================
+ Coverage   63.55%   63.58%   +0.03%     
==========================================
  Files         347      347              
  Lines       32397    32397              
==========================================
+ Hits        20589    20600      +11     
+ Misses       9261     9224      -37     
- Partials     2547     2573      +26

Impacted Files	Coverage Δ
go/worker/storage/service_internal.go	0% <0%> (-53.34%)	⬇️
go/worker/storage/api/grpc.go	20% <0%> (-22.86%)	⬇️
go/worker/keymanager/service.go	55.55% <0%> (-22.23%)	⬇️
go/worker/storage/service_external.go	47.61% <0%> (-9.53%)	⬇️
...consensus/tendermint/apps/roothash/transactions.go	51.13% <0%> (-6.82%)	⬇️
go/storage/mkvs/urkel/prefetch.go	59.32% <0%> (-3.39%)	⬇️
go/storage/metrics.go	70.53% <0%> (-2.68%)	⬇️
go/consensus/tendermint/roothash/roothash.go	68.91% <0%> (-2.63%)	⬇️
go/roothash/api/commitment/pool.go	68.43% <0%> (-2.07%)	⬇️
go/keymanager/client/client.go	73.86% <0%> (-1.51%)	⬇️
... and 11 more

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