Fix error with nil modules_dir running kafo-export-params

http://koji.katello.org/koji/getfile?taskID=73251&name=build.log

<pre>
/usr/lib/ruby/gems/1.8/gems/kafo-0.3.6/lib/kafo/puppet_module.rb:20:in `join': can't convert nil into String (TypeError)
	from /usr/lib/ruby/gems/1.8/gems/kafo-0.3.6/lib/kafo/puppet_module.rb:20:in `initialize'
	from /usr/lib/ruby/gems/1.8/gems/kafo-0.3.6/lib/kafo/configuration.rb:71:in `new'
	from /usr/lib/ruby/gems/1.8/gems/kafo-0.3.6/lib/kafo/configuration.rb:71:in `modules'
	from /usr/lib/ruby/gems/1.8/gems/kafo-0.3.6/lib/kafo/configuration.rb:71:in `map'
	from /usr/lib/ruby/gems/1.8/gems/kafo-0.3.6/lib/kafo/configuration.rb:71:in `modules'
	from /usr/lib/ruby/gems/1.8/gems/kafo-0.3.6/bin/kafo-export-params:83:in `print_out'
	from /usr/lib/ruby/gems/1.8/gems/kafo-0.3.6/bin/kafo-export-params:33:in `execute'
	from /usr/lib/ruby/gems/1.8/gems/clamp-0.6.2/lib/clamp/command.rb:67:in `run'
	from /usr/lib/ruby/gems/1.8/gems/clamp-0.6.2/lib/clamp/command.rb:125:in `run'
	from /usr/lib/ruby/gems/1.8/gems/kafo-0.3.6/bin/kafo-export-params:128
	from /usr/lib/ruby/gems/1.8/bin/kafo-export-params:19:in `load'
	from /usr/lib/ruby/gems/1.8/bin/kafo-export-params:19
</pre>