Fix empty command

Don't raise an exception if you just hit enter. I guess this is only possible through the console or a crappy IRC client.

```
> 
Error running command: undefined method `upcase' for nil:NilClass
/var/lib/gems/1.9.1/gems/tkellem-0.9.3/lib/tkellem/tkellem_bot.rb:12:in `run_command'
/var/lib/gems/1.9.1/gems/tkellem-0.9.3/lib/tkellem/socket_server.rb:23:in `receive_line'
/var/lib/gems/1.9.1/gems/eventmachine-1.0.3/lib/em/protocols/linetext2.rb:64:in `receive_data'
/var/lib/gems/1.9.1/gems/eventmachine-1.0.3/lib/eventmachine.rb:187:in `run_machine'
/var/lib/gems/1.9.1/gems/eventmachine-1.0.3/lib/eventmachine.rb:187:in `run'
/var/lib/gems/1.9.1/gems/tkellem-0.9.3/lib/tkellem/daemon.rb:92:in `start'
/var/lib/gems/1.9.1/gems/tkellem-0.9.3/lib/tkellem/daemon.rb:43:in `run'
/var/lib/gems/1.9.1/gems/tkellem-0.9.3/bin/tkellem:5:in `<top (required)>'
/usr/local/bin/tkellem:19:in `load'
/usr/local/bin/tkellem:19:in `<main>'
```