[Mono.Android] nullability for AndroidMessageHandler.RequestNeedsAuth…

…orization (#7454)

Context: https://learn.microsoft.com/dotnet/api/system.diagnostics.codeanalysis.membernotnullwhenattribute

The `MemberNotNullWhenAttribute ` annotation allows `RequestedAuthentication` to be used without a
null-check after testing, `RequestNeedsAuthorization`, when nullability analysis is enabled.

* Only apply annotation when building for .NET 7+

Co-authored-by: Jonathan Peppers <jonathan.peppers@microsoft.com>

src/Mono.Android/Xamarin.Android.Net/AndroidMessageHandler.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 using System.IO;
 using System.IO.Compression;
 using System.Linq;
@@ -233,6 +234,9 @@ public int MaxAutomaticRedirections
 		/// If <c>true</c> then the server requested authorization and the application must use information
 		/// found in <see cref="RequestedAuthentication"/> to set the value of <see cref="PreAuthenticationData"/>
 		/// </summary>
+#if NETCOREAPP
+		[MemberNotNullWhen(true, nameof(RequestedAuthentication))]
+#endif
 		public bool RequestNeedsAuthorization {
 			get { return RequestedAuthentication?.Count > 0; }
 		}