[systemcomponentmodel] Add nullability to (generated and manual) bindings (#14886)

Co-authored-by: tj_devel709 <antlambe@microsoft.com>

src/System.ComponentModel/CancelEventArgs.cs

@@ -27,6 +27,8 @@
 // WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 //
 
+#nullable enable
+
 using System;
 
 namespace System.ComponentModel {