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## Description
Closes: #8961
SDK allows InitGenesis to return with an empty validator set. In practice, the error for an empty validator set gets thrown in tendermint. To fix this,
* Add non-empty validator set check to the `mm.InitGenesis` function. This will break `simapp.Setup` because it relies on an empty validator set [#comment](https://github.com/cosmos/cosmos-sdk/pull/8909#issuecomment-804850834).
* Update `simapp.Setup` to use a single validator.
* Fix failing tests (Most of them are keeper tests).
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## Description
ref: #8961
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Following up on [#9697](https://github.com/cosmos/cosmos-sdk/pull/9697#pullrequestreview-727295733), this PR is the first step for the #8961.
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### Author Checklist
*All items are required. Please add a note to the item if the item is not applicable and
please add links to any relevant follow up issues.*
I have...
- [x] included the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [x] added `!` to the type prefix if API or client breaking change
- [x] targeted the correct branch (see [PR Targeting](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#pr-targeting))
- [x] provided a link to the relevant issue or specification
- [ ] followed the guidelines for [building modules](https://github.com/cosmos/cosmos-sdk/blob/master/docs/building-modules)
- [ ] included the necessary unit and integration [tests](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#testing)
- [x] added a changelog entry to `CHANGELOG.md`
- [ ] included comments for [documenting Go code](https://blog.golang.org/godoc)
- [ ] updated the relevant documentation or specification
- [ ] reviewed "Files changed" and left comments if necessary
- [ ] confirmed all CI checks have passed
### Reviewers Checklist
*All items are required. Please add a note if the item is not applicable and please add
your handle next to the items reviewed if you only reviewed selected items.*
I have...
- [ ] confirmed the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [ ] confirmed `!` in the type prefix if API or client breaking change
- [ ] confirmed all author checklist items have been addressed
- [ ] reviewed state machine logic
- [ ] reviewed API design and naming
- [ ] reviewed documentation is accurate
- [ ] reviewed tests and test coverage
- [ ] manually tested (if applicable)
MD Aleem
Marko
dauTT
Alexander Bezobchuk
Jonathan Gimeno