fix error when export chain state

[yihuang]

Description

To reproduce:

$ ethermintd export --height -1 --for-zero-height --home /chain/.ethermintd --chain-id cronostestnet-338
...
Error: error exporting state: expected validator ethvaloper1znla...fff7d7 not found

Related cosmos-sdk change: https://github.com/cosmos/cosmos-sdk/pull/8363/files#diff-42390a0f12b0fc40eb466b1f1b8de3b34022f112e216ac26a177435ffffba529R160

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For contributor use:

• Targeted PR against correct branch (see CONTRIBUTING.md)
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the module structure standards.
• Wrote unit and integration tests
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer

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For admin use:

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• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[codecov]

Codecov Report

Merging #446 (3d1f063) into main (2edcbf4) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##             main     #446   +/-   ##
=======================================
  Coverage   50.58%   50.58%           
=======================================
  Files          50       50           
  Lines        4928     4928           
=======================================
  Hits         2493     2493           
  Misses       2328     2328           
  Partials      107      107           

Impacted Files	Coverage Δ
app/export.go	13.88% <0.00%> (ø)