Fix bug seen when channel topic is null

[MarkRoddy]

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:inblock 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:inblock 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:inblock 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:inblock 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:inset_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:inunbind' 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:inrun_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:inrun' 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:inload' from /usr/local/bin/camper_van:19:in `

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