[msbuild] Add a check for actually having anything to AOTCompile. (#2…

…0206)

This particular condition shouldn't happen, but occasional build errors in the
wild (with exception stack traces) seem to indicate otherwise, so be a bit
defensive here.

msbuild/Xamarin.MacDev.Tasks/Tasks/AOTCompile.cs

@@ -223,6 +223,12 @@ bool IsUpToDate (FileInfo input, FileInfo output)
 
 		public override bool Execute ()
 		{
+			if (Assemblies.Length == 0) {
+				// Not sure how this can happen, since Assemblies is [Required], but it seems to happen once in a while.
+				Log.LogMessage (MessageImportance.Low, "Nothing to AOT-compile");
+				return true;
+			}
+
 			if (ShouldExecuteRemotely ())
 				return new TaskRunner (SessionId, BuildEngine4).RunAsync (this).Result;