Handle cases when authorized_keys doesn't exist

What does this MR do?

Modify Gitlab::AuthorizedKeys#rm_key and #list_key_ids to handle cases when an Errno::ENOENT is raised. Also added test cases in spec to ensure each method will still work when authorized_keys doesn't exist.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/10514

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer