Index project access levels as PRIVATE when missing project_feature

What does this MR do and why?

I kept running into indexing errors in gdk after reseeding my database. I narrowed it down to the GitLab Instance / Monitoring project (which is created without a project_feature record). I decided to refactor the project_feature indexing code and just set everything to private if the project_feature is null and updated the specs.

Screenshots or screen recordings

N/A

How to set up and validate locally

I'm not sure if this is a reliable way to reproduce because the self monitoring project doesn't always fail to index:

1. re-seed your database
2. make sure gdk is enabled for elasticsearch and that advanced search is enabled for search/indexing
3. run rake gitlab:elastic:index to create the index from scratch

another way to reproduce this is:

1. find a project with a null project_feature (or delete the record for an existing project)
2. in rails console call the as_indexed_json method: Project.find(19).__elasticsearch__.as_indexed_json
3. verify it does not throw an error and sets all of the *_access_level keys to PRIVATE (10)

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