[msbuild] Merge the CollectITunesSourceFiles and CollectITunesSourceFilesTaskBase classes. (#17912)

We no longer need two have overridable logic for remote builds, so the
non-abstract task class and the abstract base class can be merged.

Also enable nullability and fix any issues.

msbuild/Xamarin.iOS.Tasks/Tasks/CollectITunesSourceFiles.cs

@@ -1,20 +0,0 @@
-using Microsoft.Build.Framework;
-using Xamarin.Messaging.Build.Client;
-
-namespace Xamarin.iOS.Tasks {
-	public class CollectITunesSourceFiles : CollectITunesSourceFilesTaskBase, ICancelableTask {
-		public override bool Execute ()
-		{
-			if (ShouldExecuteRemotely ())
-				return new TaskRunner (SessionId, BuildEngine4).RunAsync (this).Result;
-
-			return base.Execute ();
-		}
-
-		public void Cancel ()
-		{
-			if (ShouldExecuteRemotely ())
-				BuildConnection.CancelAsync (BuildEngine4).Wait ();
-		}
-	}
-}

msbuild/Xamarin.iOS.Tasks/Tasks/CollectITunesSourceFilesTaskBase.cs

@@ -6,27 +6,33 @@ using Microsoft.Build.Utilities;
 using Microsoft.Build.Framework;
 
 using Xamarin.MacDev.Tasks;
+using Xamarin.Messaging.Build.Client;
+
+#nullable enable
 
 namespace Xamarin.iOS.Tasks {
-	public abstract class CollectITunesSourceFilesTaskBase : XamarinTask {
+	public class CollectITunesSourceFiles : XamarinTask, ICancelableTask {
 		static readonly string [] iTunesFileNames = { "iTunesMetadata.plist", "iTunesArtwork@2x", "iTunesArtwork" };
 
 		#region Inputs
 
 		[Required]
-		public string OutputPath { get; set; }
+		public string OutputPath { get; set; } = string.Empty;
 
 		#endregion
 
 		#region Outputs
 
 		[Output]
-		public ITaskItem [] ITunesSourceFiles { get; set; }
+		public ITaskItem [] ITunesSourceFiles { get; set; } = Array.Empty<ITaskItem> ();
 
 		#endregion
 
 		public override bool Execute ()
 		{
+			if (ShouldExecuteRemotely ())
+				return new TaskRunner (SessionId, BuildEngine4).RunAsync (this).Result;
+
 			var files = new List<ITaskItem> ();
 
 			foreach (var fileName in iTunesFileNames) {
@@ -42,5 +48,11 @@ namespace Xamarin.iOS.Tasks {
 
 			return !Log.HasLoggedErrors;
 		}
+
+		public void Cancel ()
+		{
+			if (ShouldExecuteRemotely ())
+				BuildConnection.CancelAsync (BuildEngine4).Wait ();
+		}
 	}
 }