Resolve "Removed branch link in pipelines page is broken"

What does this MR do?

Only renders the link to the source branch when the source branch exists.

What are the relevant issue numbers?

#42611 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [NA] 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
• [NA] Conforms to the database guides

Closes #42611 (closed)