Resolve "Provide NullStore for RequestStore"

What does this MR do?

Makes it easier and safer to use RequestStore because you don't need to check RequestStore.active? before using it. You just have to use Gitlab::SafeRequestStore instead.

For example, one consequence of forgetting to do this leads to state being leaked across tests.

What are the relevant issue numbers?

Resolves https://gitlab.com/gitlab-org/gitlab-ce/issues/51718

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides