Bug: Branch pipelines run on MRs after #1327 was merged

As part of adding the workflow:rules:changes, we missed checking for the presence of an MR on the condition, causing the changes to evaluate to true for branch pipelines, and run the jobs. We need to add an IF condition for the presence of an MR to ensure branch pipelines don't run in addition to MR pipelines.

Example: Job #3341317994 failed for 3f2061d1: