Enable nullable reference types

[AndrewKeepCoding]

⚠️DO NOT MERGE
I'll push commits per sections.

Description

This PR will enable Nullable to help catch potential null reference issues at compile time, improve code clarity, and align the samples with modern .NET best practices.

Motivation and Context

Closes #2066.

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

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

Issues not Directly Related to This PR

I'll use this section to note any unrelated issues I encounter while working on this PR.

• Scratch Pad page: Clicking the Load button throws an exception when no local settings are present.
• AnimatedIcon page: Clicking the button in the first sample and then closing the app results in an exception.
• PullToRefresh page: Performing a pull-to-refresh on the second sample, then navigating to another page, triggers an exception.
• RichEditBox page: Menu_Opening method contains duplicate code.

[marcelwgn]

Thank you for working on this!
Is it fair to assume that once a file has been "converted", you wont be touching it again? That way, I could start reviewing files and dont need to fear that the changes might be overwritten later and having to review it again.

[AndrewKeepCoding]

Yes, that's the idea. After going through all *.cs files, I'll do another pass to catch any remaining changes though they should be minor.