Fix kafka initialize when not defined for environment

Catches no-method error when attempting to call symbolize keys.
It made more sense to me to jump to the branch that sets up the null adapter by catching another error condition. Otherwise the begin statement would have to be 

```ruby
config = config.fetch(Rails.env, { })
adapter = config.empty? :null : nil
[ adapter, config ]
```

Which meant having two conditional branches (no config file and nothing defined for an environment) that do the same thing.

Thanks this just helped me catch a different error in the setup. In that the result of `config[Rails.env].symbolize_keys` isn't actually used.