User not defined in PostReceive#process_project_changes

What does this MR do?

Fails early if @user is not defined in PostReceive#process_project_changes

Does this MR meet the acceptance criteria?

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

What are the relevant issue numbers?

Closes #51335 (closed)