Follow-up from "GithubImporter: Fixes merged_by importer when merger user is nil"

The following discussion from !65152 (merged) should be addressed:

• @georgekoltsov started a discussion:

  non blocking: should 'ghost' be a constant of sorts that could be reused in multiple places where it is needed? I imagine this can be the case of not only merged by, but also PR reviews, issues, etc.