Fix bug seen when channel topic is null

When the topic for a channel is null, the process crashes due to TypeError (converting nil to string).  See full stack trace below.



/var/lib/gems/1.9.1/gems/camper_van-0.0.6/lib/camper_van/channel.rb:59:in `+': can't convert nil into String (TypeError)
        from /var/lib/gems/1.9.1/gems/camper_van-0.0.6/lib/camper_van/channel.rb:59:in `block in join'
        from /var/lib/gems/1.9.1/gems/camper_van-0.0.6/lib/camper_van/channel.rb:240:in `call'
        from /var/lib/gems/1.9.1/gems/camper_van-0.0.6/lib/camper_van/channel.rb:240:in `block in update_users'
        from /var/lib/gems/1.9.1/gems/firering-1.2.2/lib/firering/data/room.rb:22:in `call'
        from /var/lib/gems/1.9.1/gems/firering-1.2.2/lib/firering/data/room.rb:22:in `block in users'
        from /var/lib/gems/1.9.1/gems/firering-1.2.2/lib/firering/connection.rb:69:in `call'
        from /var/lib/gems/1.9.1/gems/firering-1.2.2/lib/firering/connection.rb:69:in `block in http'
        from /var/lib/gems/1.9.1/gems/eventmachine-0.12.10/lib/em/deferrable.rb:134:in `call'
        from /var/lib/gems/1.9.1/gems/eventmachine-0.12.10/lib/em/deferrable.rb:134:in `set_deferred_status'
        from /var/lib/gems/1.9.1/gems/eventmachine-0.12.10/lib/em/deferrable.rb:173:in `succeed'
        from /var/lib/gems/1.9.1/gems/em-http-request-0.3.0/lib/em-http/client.rb:307:in `unbind'
        from /var/lib/gems/1.9.1/gems/eventmachine-0.12.10/lib/eventmachine.rb:1417:in `event_callback'
        from /var/lib/gems/1.9.1/gems/eventmachine-0.12.10/lib/eventmachine.rb:256:in `run_machine'
        from /var/lib/gems/1.9.1/gems/eventmachine-0.12.10/lib/eventmachine.rb:256:in `run'
        from /var/lib/gems/1.9.1/gems/camper_van-0.0.6/lib/camper_van/server.rb:21:in `run'
        from /var/lib/gems/1.9.1/gems/camper_van-0.0.6/bin/camper_van:64:in `<top (required)>'
        from /usr/local/bin/camper_van:19:in `load'
        from /usr/local/bin/camper_van:19:in `<main>'