Fix NoMethodError for calling modified_paths on nil pipeline

What does this MR do and why?

• Contributes to #351520 (closed)
• Sentry error: https://sentry.gitlab.net/gitlab/gitlabcom/issues/3166948

Problem

We don't expect the case when a pipeline is nil while explicitly calling the method with nil in the execution tree

Solution

State that the rule is satisfied when a pipeline is nil