Fix error message when removing only child from GraphNode

[Santoss1809]

Pull Request for issue regarding error thrown on delete of only child of a graph node.

Bugsquad edit:

• Fixes Error message when removing only child from GraphNode #89695

[Geometror]

Thanks for looking into this!
There's just one thing: Please remove the issue reference in your commit message to prevent spamming the issue with reference updates :)

[akien-mga]

@teacher: I met your student today and confirm this was a top-notch first contribution to an open source project of the scale of Godot ;)

(Also do reach out, I'm interested in your program to get students contributing to open source ;))

[akien-mga]

Amended the commit to address review feedback.

[akien-mga]

Thanks! And congrats for your first merged Godot contribution 🎉