Core - Change Cef.IsInitialized from bool to bool?

[amaitland]

Fixes:

4867

Summary: Change to nullable bool

Changes: [specify the structures changed]

• change type
• Throw exception if false

How Has This Been Tested?
Needs more testing

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [x] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Updated documentation

Checklist:

• [ ] Tested the code(if applicable)
• [ ] Commented my code
• [ ] Changed the documentation(if applicable)
• [ ] New files have a license disclaimer
• [x] The formatting is consistent with the project (project supports .editorconfig)

[AppVeyorBot]

:white_check_mark: Build CefSharp 126.2.70-CI5031 completed (commit https://github.com/cefsharp/CefSharp/commit/49b0d89047 by @amaitland)