Issue: git-open-pr step fails when no changes exist between source and target branches

Issue: git-open-pr step fails when no changes exist between source and target branches: a task in LegoFlow-SWE (Harbor dataset). Observed behavior When a promotion pipeline (e.g., DEV → SIT → UAT → PROD) promotes a freight that doesn’t contain any commits that differ between the source and target…

The task

**Observed behavior** When a promotion pipeline (e.g., DEV → SIT → UAT → PROD) promotes a freight that doesn’t contain any commits that differ between the source and target branches, the `git-open-pr` step fails with a git provider validation error similar to:

Part of Lego-X/LegoFlow-SWE.