fix: Make engine parameter optional for scene constructor

[miqh]

Hello,

This minor change should address #1363.

As additional information, I noticed there aren't any usages within the codebase which invoke the parameter-less constructor for Scene. However, there are usages with the Engine argument supplied scattered throughout spec and test files. The usages of new Scene() appear to be confined to the documentation.

I also noticed there had been a change made in the past (#1068) which caused the Engine argument to be mandatory in the constructor signature to begin with. It's not clear to me exactly why that was done given the context available.

---

===:clipboard: PR Checklist :clipboard:===

• 📌 issue exists in github for these changes
• 🔬 existing tests still pass
• 🙈 code conforms to the style guide
• 📐 new tests written and passing / old tests updated with new scenario(s)
• 📄 changelog entry added (or not needed)

==================

[eonarheim]

@miqh Thanks for your contribution!

[eonarheim]

@miqh I think that the change to make it mandatory introduced in #1068 was an oversight on our part.