Empty rollapp genesis state breaks the genesis event

[mtsitrin]

          if genesis state remains `nil` I think we would get an error in the genesis event later on:

https://github.com/dymensionxyz/dymension/blob/main/x/rollapp/keeper/keeper.go#L68
Not sure if that's expected, if not, we should instantiate rollappGenesisState regardless of genesis accounts being empty or not.
Unless it's expected that for some reason we enforce genesis accounts not being empty. In that case, we should probably fail on validation or something.

Originally posted by @zale144 in #712 (comment)