Fix division by zero for blame age map

What does this MR do?

Fixes a division by zero error when the project is less than 1 day old.

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

None.

Why was this MR needed?

Viewing blame page on project less than 1 day old results in error.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• N/A Documentation created/updated
• N/A API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #35441 (closed)