Debian Support

I noticed this while working with remotes, but it may well affect other things as well.

Stepos to reproduce:

1. Create an example remote:

   http post :/pulp/api/v3/remotes/deb/apt/ name=tuxedo_remote distributions="bionic" components="main"architectures="i386 amd64" url="http://deb.tuxedocomputers.com/ubuntu/"
   

2. Try to update the components to null:

   $ http patch :/pulp/api/v3/remotes/deb/apt/<uuid>/ components:=null --json
   HTTP/1.1 400 Bad Request
   Allow: GET, PUT, PATCH, DELETE, HEAD, OPTIONS
   Connection: keep-alive
   Content-Length: 46
   Content-Type: application/json
   Date: Thu, 04 Jun 2020 11:55:12 GMT
   Server: nginx/1.14.2
   Vary: Accept, Cookie
   X-Frame-Options: SAMEORIGIN
   
   {
       "components": [
           "This field may not be null."
       ]
   }
   

Note that it is perfectly legal to create the remote with the null field by simply omitting that option. Functionally it makes perfect sense to change your mind about this later. It should be allowed to null such nullable fields!