[Task]: Handle 204 responses returned by webhook scanners

[willdurand]

Description

Once we have #16097 fixed, we'll be able to mark a scanner result as "skipped" by setting result to None when a scanner result returned a 204 HTTP response. We'll want to document that, too.

Acceptance Criteria

see above

┆Issue is synchronized with this Jira Task

[willdurand]

For QA: submitting an add-on with a customs license should create a scanner result for the source builder with no results (in the django admin). I'd expect "-" instead of "[]".

[ioanarusiczki]

I see "None" on this result.

A new version with source code at upload time for this addon with custom license -> "None" again for on_source_code_uploaded and on_version_created.

I hope that's fine.