EE Resolve "`show_gold_trial?` fails when not logged in"

What does this MR do?

CE: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23526

There is currently an issue on the explore pages, they can be viewed by users that aren't logged in and our conditional to render the gitlab.com gold trial callout depends on a user account and will raise an exception if it is nil.

This MR checks for the user record before continuing to check if that user has any namespaces that have a gold plan or active trial.

What are the relevant issue numbers?

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
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer

Closes #8684 (closed)