fix a few things

Author: dellis1972

src/Xamarin.Android.Build.Tasks/Tasks/Aapt2Compile.cs

@@ -37,7 +37,7 @@ public class Aapt2Compile : Aapt2 {
 
 		protected override int GetRequiredDaemonInstances ()
 		{
-			return Math.Min ((ResourcesToCompile ?? ResourceDirectories).Length, DaemonMaxInstanceCount);
+			return Math.Min ((ResourcesToCompile ?? ResourceDirectories)?.Length ?? 1, DaemonMaxInstanceCount);
 		}
 
 		public async override System.Threading.Tasks.Task RunTaskAsync ()
@@ -67,7 +67,7 @@ void ProcessDirectory (ITaskItem item, object lockObject)
 			if (string.IsNullOrEmpty (fileOrDirectory) || !isDirectory)
 				fileOrDirectory = item.ItemSpec;
 			if (isDirectory && !Directory.Exists (fileOrDirectory)) {
-				LogWarning ($"Ignoring '{fileOrDirectory}' as it does not exist!");
+				LogWarning ($"Ignoring directory '{fileOrDirectory}' as it does not exist!");
 				return;
 			}
 			if (isDirectory && !Directory.EnumerateDirectories (fileOrDirectory).Any ())