Fix nil position error in UpdateDiffPositionService

What does this MR do?

Fixes a NoMethodError that occurs when UpdateDiffPositionService tries to create a system note for outdated diff discussions with a nil position.

https://log.gprd.gitlab.net/app/r/s/bLbsG

Related issues

Resolves the error: undefined method 'diff_refs' for nil:NilClass in the stack trace provided.

How to test

This error occurs during merge request diff updates when the position tracer returns a nil position. The fix prevents the system note creation when position is nil.

What are the relevant issue numbers?

N/A - Bug fix based on error stack trace

Does this MR meet the acceptance criteria?

• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guidelines (if applicable)
• Separation of EE specific content (if applicable)

Availability and Testing

• Review by a SET
• Added tests
• End-to-end tests

Risk Assessment

Low risk - This is a defensive fix that prevents a crash by adding a nil check. The behavior when position is nil is to skip creating the system note, which is the appropriate fallback behavior.