Fix stubbing attributes alongside cache_markdown_field

What does this MR do?

Regenerates the HTML for markdown fields if there is text in the markdown field and a blank string in the corresponding HTML field.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Previously, we'd only regenerate the HTML if the html field was nil. This turned out to break some specs when stubbing the relevant text fields, e.g., https://gitlab.com/gitlab-org/gitlab-ce/issues/34698#note_34263270

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #34698 (closed)