🚧Remove nullable reference in scene manager

Complete The Item Below

[X] I have updated the title without removing the 🚧 emoji.

Description

In the internal SceneManager class, the list of scenes contains a tuple where the IScene in that tuple is a nullable reference type.

This should not be the case. Refactor the code in the SceneManager class to remove the IScene as a nullable reference type.

Acceptance Criteria

### The items to complete to satisfy the Definition of Done.
- [ ] All unit tests executed and passed locally after PR work is complete. _(Why? Refer to 'Additional Information' below.)_
- [ ] Nullable reference type changed to non-nullable reference type

ToDo Items

### The items to complete to satisfy the Definition of Done.
- [x] Change type labels added to this issue.  Refer to the _**Change Type Labels**_ section below.
- [x] Priority label added to this issue.  Refer to the _**Priority Type Labels**_ section below.
- [x] Issue linked to the correct milestone _(if applicable)_.

Issue Dependencies

Related Work

No response

Additional Information:

Unit Tests

Reasons for local unit test execution:

Unit tests might pass locally but not in the CI environment during the status check process or vice-versa.
Tests might pass on the developer's machine but not necessarily on the code reviewer's machine.
If you notice that the test status check has passed but the test failed locally, please notify a project maintainer!

💡Warning💡 If the unit tests pass remotely and are not executed locally, this means we could be letting a bug slip into production. Though bugs will always exist in some capacity, we should all do our part to help prevent them from happening.

Change Type Labels

Change Type	Label
Bug Fixes	`🐛bug`
Breaking Changes	`🧨breaking changes`
New Feature	`✨new feature`
CICD Changes	`♻️cicd`
Config Changes	`⚙️config`
Performance Improvements	`🏎️performance`
Code Doc Changes	`🗒️documentation/code`
Product Doc Changes	`📝documentation/product`

Priority Type Labels

Priority Type	Label
Low Priority	`low priority`
Medium Priority	`medium priority`
High Priority	`high priority`

Code of Conduct

[X] I agree to follow this project's Code of Conduct.

KinsonDigital / Velaptor