Bugfix - list_public_ip_addresses() may return Nil

Bug fix - method list_public_ip_addresses() may return Nil what result in the following error:
knife cs server list -VV
/opt/chef-server/embedded/lib/ruby/gems/1.9.1/gems/knife-cloudstack-0.0.13/lib/knife-cloudstack/connection.rb:84:in `get_server_public_ip': undefined method `find' for nil:NilClass (NoMethodError)
        from /opt/chef-server/embedded/lib/ruby/gems/1.9.1/gems/knife-cloudstack-0.0.13/lib/chef/knife/cs_server_list.rb:92:in `block in run'
        from /opt/chef-server/embedded/lib/ruby/gems/1.9.1/gems/knife-cloudstack-0.0.13/lib/chef/knife/cs_server_list.rb:84:in `each'
        from /opt/chef-server/embedded/lib/ruby/gems/1.9.1/gems/knife-cloudstack-0.0.13/lib/chef/knife/cs_server_list.rb:84:in `run'
        from /opt/chef-server/embedded/lib/ruby/gems/1.9.1/gems/chef-11.4.0/lib/chef/knife.rb:460:in `run_with_pretty_exceptions'
        from /opt/chef-server/embedded/lib/ruby/gems/1.9.1/gems/chef-11.4.0/lib/chef/knife.rb:173:in `run'
        from /opt/chef-server/embedded/lib/ruby/gems/1.9.1/gems/chef-11.4.0/lib/chef/application/knife.rb:123:in `run'
        from /opt/chef-server/embedded/lib/ruby/gems/1.9.1/gems/chef-11.4.0/bin/knife:25:in `<top (required)>'
        from /opt/chef-server/embedded/bin/knife:23:in `load'
        from /opt/chef-server/embedded/bin/knife:23:in `<main>'