staking: Don't emit zero-amount staking events

[abukosek]

If fees were set to 0, staking events related to the fee accumulator
would still get emitted with zero amounts, which is pointless.

This fix affects only events related to the internal fee accumulator
and common pool accounts, manual transfers with 0 as the amount will
still get emitted.

[codecov]

Codecov Report

Merging #2983 into master will increase coverage by 0.25%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #2983      +/-   ##
==========================================
+ Coverage   68.51%   68.77%   +0.25%     
==========================================
  Files         360      360              
  Lines       35386    35379       -7     
==========================================
+ Hits        24246    24333      +87     
+ Misses       8035     7936      -99     
- Partials     3105     3110       +5     

Impacted Files	Coverage Δ
go/staking/tests/tester.go	92.12% <40.00%> (-1.87%)	⬇️
go/consensus/tendermint/apps/staking/fees.go	47.44% <100.00%> (ø)
go/consensus/tendermint/apps/staking/state/gas.go	80.00% <100.00%> (-3.34%)	⬇️
go/oasis-node/cmd/common/metrics/disk.go	65.51% <0.00%> (-20.69%)	⬇️
go/consensus/tendermint/api/errors.go	86.66% <0.00%> (-13.34%)	⬇️
go/oasis-node/cmd/common/metrics/resource.go	84.00% <0.00%> (-8.00%)	⬇️
go/worker/storage/service_external.go	47.31% <0.00%> (-4.31%)	⬇️
...consensus/tendermint/apps/roothash/transactions.go	55.43% <0.00%> (-2.18%)	⬇️
...consensus/tendermint/apps/scheduler/state/state.go	71.15% <0.00%> (-1.93%)	⬇️
... and 27 more

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