Fix nullpointerexception in lib/webistrano/deployer.rb

The error occured when an empty config value was specified. This
happened in e.g. this deployment:
http://webistrano.enrise.com/projects/25/stages/132/deployments/5917

*** can't dup NilClass
*** /srv/http/webistrano/releases/1370357193/lib/webistrano/deployer.rb:154:in `dup'
*** /srv/http/webistrano/releases/1370357193/lib/webistrano/deployer.rb:154:in `resolve_references'
*** /srv/http/webistrano/releases/1370357193/lib/webistrano/deployer.rb:144:in `set_stage_configuration'
*** /srv/http/webistrano/releases/1370357193/lib/webistrano/deployer.rb:143:in `each'
*** /srv/http/webistrano/releases/1370357193/lib/webistrano/deployer.rb:143:in `set_stage_configuration'
*** /srv/http/webistrano/releases/1370357193/lib/webistrano/deployer.rb:122:in `set_up_config'
*** /srv/http/webistrano/releases/1370357193/lib/webistrano/deployer.rb:68:in `execute!'
*** /srv/http/webistrano/releases/1370357193/lib/webistrano/deployer.rb:65:in `catch'
*** /srv/http/webistrano/releases/1370357193/lib/webistrano/deployer.rb:65:in `execute!'
*** /srv/http/webistrano/releases/1370357193/lib/webistrano/deployer.rb:49:in `invoke_task!'
*** (eval):1
*** script/runner:3:in `eval'
*** /srv/http/webistrano/releases/1370357193/vendor/bundle/ruby/1.8/gems/rails-2.3.14/lib/commands/runner.rb:48
*** script/runner:3:in `require'
*** script/runner:3