fix(rollapp): genesisState of rollapp is non-nullable struct

[mtsitrin]

Description

setting RollappGenesisState as non nullable object, as it's always required.
the repeated GenesisAccount is nullable

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Closes #740

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.

PR review checkboxes:

I have...

• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• Targeted PR against the correct branch
• included the correct type prefix in the PR title
• Linked to the GitHub issue with discussion and accepted design
• Targets only one GitHub issue
• Wrote unit and integration tests
• Wrote relevant migration scripts if necessary
• All CI checks have passed
• Added relevant godoc comments
• Updated the scripts for local run, e.g genesis_config_commands.sh if the PR changes parameters
• Add an issue in the e2e-tests repo if necessary

SDK Checklist

• Import/Export Genesis
• Registered Invariants
• Registered Events
• Updated openapi.yaml
• No usage of go map
• No usage of time.Now()
• Used fixed point arithmetic and not float arithmetic
• Avoid panicking in Begin/End block as much as possible
• No unexpected math Overflow
• Used sendCoin and not SendCoins
• Out-of-block compute is bounded
• No serialized ID at the end of store keys
• UInt to byte conversion should use BigEndian

Full security checklist here

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For Reviewer:

• Confirmed the correct type prefix in the PR title
• Reviewers assigned
• Confirmed all author checklist items have been addressed

---;

After reviewer approval:

• In case the PR targets the main branch, PR should not be squash merge in order to keep meaningful git history.
• In case the PR targets a release branch, PR must be rebased.

[zale144]

Thanks!

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 7 lines in your changes are missing coverage. Please review.

Project coverage is 31.20%. Comparing base (bc7ad7f) to head (99598ad).

Files	Patch %	Lines
x/rollapp/types/rollapp.pb.go	64.28%	4 Missing and 1 partial ⚠️
x/rollapp/types/tx.pb.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #742      +/-   ##
==========================================
- Coverage   31.23%   31.20%   -0.03%     
==========================================
  Files         233      233              
  Lines       32593    32582      -11     
==========================================
- Hits        10179    10167      -12     
  Misses      20782    20782              
- Partials     1632     1633       +1     

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