Revert headerTime nullish coalescing

What does this MR do?

This reverts the changes to the headerTime getter in job logs, introduced in !52825 (merged), as this throws warnings on the console

#322554 (closed)

Screenshots (strongly suggested)

N/A

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