Description of problem:
unable to set "Default HTTP Proxy" field to "no global default".

Version-Release number of selected component (if applicable):
- Satellite 6.9.0 snap 9

How reproducible:
- Always

Steps to Reproduce:
1. Create a Http Proxy (Go to Infrastructure > Http Proxies)
2. Go to Administer > Settings > Content tab
3. Set "Default HTTP Proxy"
4. Now try to set "Default HTTP Proxy" to "no global default".

Actual results:
Oh no! Something went wrong while submitting the form, the server returned the following error: Error: Request failed with status code 422

Expected results:
- User is able to set "Default HTTP Proxy" to "no global default".

Additional info:

2021-01-20T03:42:34 [E|app|9a9bad14] Unprocessable entity Setting::Content (id: 198):
 9a9bad14 |   There is no such HTTP Proxy
 9a9bad14 | 
2021-01-20T03:42:34 [I|app|9a9bad14]   Rendering api/v2/errors/unprocessable_entity.json.rabl within api/v2/layouts/error_layout
2021-01-20T03:42:34 [I|app|9a9bad14]   Rendered api/v2/errors/unprocessable_entity.json.rabl within api/v2/layouts/error_layout (Duration: 4.8ms | Allocations: 6575)
2021-01-20T03:42:34 [D|app|9a9bad14] Body: {
 9a9bad14 |   "error": {"id":198,"errors":{"base":["There is no such HTTP Proxy"]},"full_messages":["There is no such HTTP Proxy"]}
 9a9bad14 | }
 9a9bad14 | 
2021-01-20T03:42:34 [I|app|9a9bad14] Completed 422 Unprocessable Entity in 26ms (Views: 8.4ms | ActiveRecord: 3.1ms | Allocations: 18645)


Workaround:
- Use hammer CLI to unset content_default_http_proxy
hammer> settings set --name content_default_http_proxy --value ""