Ignore empty ROLLBAR_ENV
Ignore empty ROLLBAR_ENV

If ENV variables are not set, they appear as `nil` in ruby when accessed. If they are empty, they appear as en empty string `""` to ruby. An empty strings is true when used with `||` operator in ruby. As we want to have an environment set for Rollbar, we should use .presence to ignore empty ROLLBAR_ENV variables and use the Rails.env instead.

```bash
$ ENV_FOO=test ENV_BAR= be rails c
Loading development environment (Rails 5.0.3)
2.3.3 :001 > ENV['ENV_FOO']
 => "test" 
2.3.3 :002 > ENV['ENV_BAR']
 => "" 
2.3.3 :003 > ENV['ENV_BAZ']
 => nil 
```
rollbar/rollbar-gem,rollbar/rollbar-gem,rollbar/rollbar-gem