allows plugins without stream specific configuration to be added

Before, when an output plugin that didn't have any stream specific configuration (stream_configuration was {}) we would skip adding any fields to the form, including description. This makes `streams_controller#create_output` choke.

Now at the beginning we just filter out all outputs with a nil stream_configuration. The only outputs that ever should meet this condition are ones that were added while graylog2-server has been running. When the server restarts it will write in {} or whatever the plugin desires and all will be right with the world.