Ensure diff array isn't nil before continuing

What does this Merge Request do and why?

This MR fixes an issue where the output of git diff is empty.

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.

Closes #1361 (closed)