Fix GitLab::JsonCache when reading a persisted entry back from the cache

What does this MR do?

Port changes in GitLab::JsonCache class on https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9732 to CE.

What are the relevant issue numbers?

1. https://gitlab.com/gitlab-org/gitlab-ee/issues/9903
2. https://gitlab.com/gitlab-org/gitlab-ee/issues/10041

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