fix(staking): Fix JSON genesis migration

[amaury1093]

Description

ref: #11880

TEST:

./build/v045_node export > old.json
./build/v046_node migrate v0.46 old.json > new.json
./build/v046_node init alice
cp new.json ~/.simapp/config/genesis.json
./build/v046_node start --mode validator

The above script panicked on the start command before this PR.

---

Author Checklist

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• confirmed the correct type prefix in the PR title
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• reviewed state machine logic
• reviewed API design and naming
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[codecov]

Codecov Report

Merging #12140 (b5236cf) into main (889dfcb) will decrease coverage by 0.03%.
The diff coverage is 76.96%.

@@            Coverage Diff             @@
##             main   #12140      +/-   ##
==========================================
- Coverage   66.04%   66.00%   -0.04%     
==========================================
  Files         671      672       +1     
  Lines       71055    71044      -11     
==========================================
- Hits        46927    46896      -31     
- Misses      21465    21485      +20     
  Partials     2663     2663              

Impacted Files	Coverage Δ
baseapp/baseapp.go	79.27% <ø> (ø)
client/cmd.go	57.73% <0.00%> (ø)
client/grpc_query.go	30.33% <0.00%> (ø)
client/keys/add.go	63.34% <0.00%> (ø)
pruning/manager.go	95.18% <ø> (ø)
server/mock/tx.go	63.15% <ø> (ø)
server/rosetta/converter.go	56.28% <0.00%> (ø)
simapp/test_helpers.go	26.34% <0.00%> (ø)
testutil/network/network.go	62.40% <ø> (ø)
types/address.go	66.76% <ø> (ø)
... and 52 more

[alexanderbez]

utACK