Docker network (#331)

* fixes puppet run failures with no IPAM driver

* fixing variables so nil values dont cause failures

Author: davejrt

lib/puppet/provider/docker_network/ruby.rb

@@ -38,10 +38,10 @@ def self.instances
       _, name, driver = line.split(' ')
       inspect = docker(['network', 'inspect', name])
       obj = JSON.parse(inspect).first
-      ipam_driver = unless obj['IPAM']['Driver'].empty?
+      ipam_driver = unless obj['IPAM']['Driver'].nil?
                       obj['IPAM']['Driver']
                     end
-      subnet = unless obj['IPAM']['Config'].empty?
+      subnet = unless obj['IPAM']['Config'].nil?
                  if obj['IPAM']['Config'].first.key? 'Subnet'
                    obj['IPAM']['Config'].first['Subnet']
                  end