Don't crash when event has no tags

This is a fix for a crash (see below) when an event doesn't contain any tags.

```
Exception in thread "LogStash::Runner" org.jruby.exceptions.RaiseException: (NoMethodError) undefined method `join' for nil:NilClass
	at RUBY.format_message(file:/opt/logstash/agent/lib/logstash-1.3.3.jar!/logstash/outputs/s3.rb:349)
	at RUBY.receive(file:/opt/logstash/agent/lib/logstash-1.3.3.jar!/logstash/outputs/s3.rb:300)
	at RUBY.handle(file:/opt/logstash/agent/lib/logstash-1.3.3.jar!/logstash/outputs/base.rb:86)
	at RUBY.initialize((eval):24)
	at org.jruby.RubyProc.call(org/jruby/RubyProc.java:271)
	at RUBY.output(file:/opt/logstash/agent/lib/logstash-1.3.3.jar!/logstash/pipeline.rb:259)
	at RUBY.outputworker(file:/opt/logstash/agent/lib/logstash-1.3.3.jar!/logstash/pipeline.rb:218)
	at RUBY.start_outputs(file:/opt/logstash/agent/lib/logstash-1.3.3.jar!/logstash/pipeline.rb:145)
```