Steps to Reproduce:
1. Create a custom fact with a value:

Facter.add(:custom_fact) do
setcode do
'not_empty'
end
end

2. Check the facter output on the host

[root@host72 ~]# facter -p | grep custom_fact
custom_fact => not_empty

3. Run puppet agent to report the custom fact.

4. Check the hammer host facts output: (or the facts from the foreman webui for the same host)

[root@foreman ~]# hammer host facts --name host72.example.com | grep custom_fact

5. Change the fact value

Facter.add(:custom_fact) do
setcode do
''
end
end

6. Check facter output on the host. It should now report a null value.

[root@host72 ~]# facter -p | grep custom
custom_fact =>

7. Run puppet agent to report the custom fact

8. Check the hammer host facts output: (or the facts from the foreman webui for the same host)

[root@foreman ~]# hammer host facts --name host72.example.com | grep custom_fact
[root@foreman ~]

So the custom fact key:value gets completely discarded.

Actual results:

custom facts get discarded completely

Expected results:

Foreman should indicate that custom fact has a null value. Something like:

[root@foreman ~]# hammer host facts --name host72.example.com | grep custom_fact
custom_fact | null