Validator Voting: check `migrate_msg` content to be >0

[ueco-jb]

Since we allow migrate_msg field to be empty, we could actually make it an Option to not confuse frontend.
https://github.com/confio/poe-contracts/blob/main/contracts/tgrade-validator-voting/src/msg.rs#L80

    MigrateContract {
...
        /// encoded message to be passed to perform the migration
        migrate_msg: Option<Binary>,
    },

@ethanfrey to confirm

[ethanfrey]

No. We do not allow empty slice.
We allow {} (which is the serialization of EmptyMsg).

You probably need to add this base64 encoded

We should enforce length > 0.
This will fail on upload with ""