Fix nil when deleting vapp.

I got this error when trying to delete a vapp, and net_id was an empty string, it wasn't nil.

```
/opt/chefdk/embedded/lib/ruby/gems/2.1.0/gems/nokogiri-1.6.4.1/lib/nokogiri/xml/node_set.rb:214:in `attr': undefined method `attribute' for nil:NilClass (NoMethodError)
	from /Users/joaovale/.chefdk/gem/ruby/2.1.0/gems/vcloud-rest-1.3.0/lib/vcloud-rest/vcloud/vapp.rb:42:in `block in get_vapp'
	from /Users/joaovale/.chefdk/gem/ruby/2.1.0/gems/vcloud-rest-1.3.0/lib/vcloud-rest/vcloud/vapp.rb:36:in `collect'
	from /Users/joaovale/.chefdk/gem/ruby/2.1.0/gems/vcloud-rest-1.3.0/lib/vcloud-rest/vcloud/vapp.rb:36:in `get_vapp'
	from /Users/joaovale/.chefdk/gem/ruby/2.1.0/gems/vcloud-rest-1.3.0/lib/vcloud-rest/vcloud/vapp.rb:127:in `block in get_vapp_by_name'
	from /Users/joaovale/.chefdk/gem/ruby/2.1.0/gems/vcloud-rest-1.3.0/lib/vcloud-rest/vcloud/vapp.rb:125:in `each'
	from /Users/joaovale/.chefdk/gem/ruby/2.1.0/gems/vcloud-rest-1.3.0/lib/vcloud-rest/vcloud/vapp.rb:125:in `get_vapp_by_name'
	from /Users/joaovale/.chefdk/gem/ruby/2.1.0/gems/knife-vcloud-1.3.0/lib/chef/knife/common/vc_vapp_common.rb:42:in `get_vapp'
	from /Users/joaovale/.chefdk/gem/ruby/2.1.0/gems/knife-vcloud-1.3.0/lib/chef/knife/vapp/vc_vapp_delete.rb:34:in `run'
	from /opt/chefdk/embedded/apps/chef/lib/chef/knife.rb:493:in `run_with_pretty_exceptions'
	from /opt/chefdk/embedded/apps/chef/lib/chef/knife.rb:174:in `run'
	from /opt/chefdk/embedded/apps/chef/lib/chef/application/knife.rb:139:in `run'
	from /opt/chefdk/embedded/apps/chef/bin/knife:25:in `<top (required)>'
	from /usr/bin/knife:34:in `load'
	from /usr/bin/knife:34:in `<main>'
```