go/storage/mkvs: Don't fail sanity check for equal roots

[kostko]

In case the state root did not change, a write log is empty and so the
sanity check should not require that it exists.

[codecov]

Codecov Report

Merging #4381 (ae69a9b) into master (386130b) will decrease coverage by 0.43%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master    #4381      +/-   ##
==========================================
- Coverage   69.03%   68.60%   -0.44%     
==========================================
  Files         411      411              
  Lines       47436    47437       +1     
==========================================
- Hits        32749    32543     -206     
- Misses      10647    10848     +201     
- Partials     4040     4046       +6     

Impacted Files	Coverage Δ
go/storage/mkvs/db/badger/check.go	63.28% <50.00%> (+0.28%)	⬆️
go/ias/http/http.go	20.33% <0.00%> (-44.07%)	⬇️
go/oasis-node/cmd/common/metrics/disk.go	65.51% <0.00%> (-20.69%)	⬇️
go/keymanager/api/policy_sgx.go	33.33% <0.00%> (-13.34%)	⬇️
go/runtime/host/sandbox/sandbox.go	67.03% <0.00%> (-9.90%)	⬇️
go/oasis-node/cmd/common/metrics/resource.go	84.00% <0.00%> (-8.00%)	⬇️
go/oasis-node/cmd/ias/auth.go	68.96% <0.00%> (-6.90%)	⬇️
go/common/sgx/aesm/aesm.go	63.63% <0.00%> (-6.37%)	⬇️
go/consensus/tendermint/apps/beacon/genesis.go	65.71% <0.00%> (-5.72%)	⬇️
go/runtime/host/protocol/connection.go	62.16% <0.00%> (-3.87%)	⬇️
... and 42 more

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