MsgCreateValidator should not allow a self delegation of 0 consensus power

[colin-axner]

Summary

The MsgCreateValidator does not check if the PotentialConsensusPower of the self delegation is 0. It checks if the self delegation is positive, but if the self delegation is less than the PowerReduction, the consensus power will be 0. This results in the validator set being nil after running gentx with too low of an amount.

The error returned in this situation:

Error: error during handshake: error on replay: validator set is nil in genesis and still empty after InitChain

This error is very confusing for users since the gentx was clearly ran and the msg create validator is clearly included in genesis. It is pretty hard to debug as well

Alternatively, we could just add this check to the client code if for some reason this modification isn't safe to make in x/staking

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[colin-axner]

So we cannot change the minimum self delegation requirements since this could cause existing validators to have invalid minimum self delegation amounts. There's probably a way to fix it, but I'll let someone else look into it

We can change the create validator message to require an initial delegation of at least one consensus power. This seems reasonable to me