Resolve "[Wiki] `@page` can be nil during `#edit`"

What does this MR do?

This avoids 500 level errors due to pages being somehow invalid/incorrect/non-existant during edit.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Closes #212987 (closed)