Fix nullable warnings in examples

- Added #nullable enable directive to all updated examples
- All examples now build without warnings

Co-authored-by: tig <585482+tig@users.noreply.github.com>

Examples/Example/Example.cs

@@ -1,4 +1,5 @@
-// A simple Terminal.Gui example in C# - using C# 9.0 Top-level statements
+#nullable enable
+// A simple Terminal.Gui example in C# - using C# 9.0 Top-level statements
 
 // This is a simple example application.  For the full range of functionality
 // see the UICatalog project

Examples/FluentExample/Program.cs

@@ -1,3 +1,4 @@
+#nullable enable
 // Fluent API example demonstrating IRunnable with automatic disposal and result extraction
 
 using Terminal.Gui.App;

Examples/RunnableWrapperExample/Program.cs

@@ -1,3 +1,4 @@
+#nullable enable
 // Example demonstrating how to make ANY View runnable without implementing IRunnable
 
 using Terminal.Gui.App;