[chef] node attribute initialized with empty array reset to nil in recipe in Chef 11

[Bryan Berry < >]

Dear Chefs,

I am having some pain w/ the Chef 11 migration. The following code
works fine w/ Chef 10.* but fails on chef 11.*

https://github.com/lusis/chef-logstash/blob/master/attributes/default.rb#L4
default['logstash']['join_groups'] = []

https://github.com/lusis/chef-logstash/blob/master/recipes/default.rb#L31
node['logstash']['join_groups'].each do |grp|
  group grp do
    members node['logstash']['user']
    action :modify
    append true
    only_if "grep -q '^#{grp}:' /etc/group"
  end

Here is the error I get.  Apparently it somehow is set to nil .   What
am i doing wrong?

NoMethodError

-------------

undefined method `each' for nil:NilClass

Cookbook Trace:

---------------

  /etc/vagrant-chef/chef-solo-1/cookbooks/logstash/recipes/default.rb:31:in
`from_file'
  /etc/vagrant-chef/chef-solo-1/cookbooks/fao-logstash/recipes/server.rb:10:in
`from_file'

Relevant File Content:
----------------------
/etc/vagrant-chef/chef-solo-1/cookbooks/logstash/recipes/default.rb:

 24:  directory node['logstash']['basedir'] do
 25:    action :create
 26:    owner "root"
 27:    group "root"
 28:    mode "0755"
 29:  end
 30:
 31>> node['logstash']['join_groups'].each do |grp|
 32:    group grp do
 33:      members node['logstash']['user']
 34:      action :modify
 35:      append true