CompileAvaloniaXamlTask - handle no-pdb compilations (#15509)

* CompileAvaloniaXamlTask - handle no-pdb compilations

* handle PDBs separately

src/Avalonia.Build.Tasks/CompileAvaloniaXamlTask.cs

@@ -38,7 +38,7 @@ namespace Avalonia.Build.Tasks
             {
                 // To simplify incremental build checks, copy the input files to the expected output locations even if the Xaml compiler didn't do anything.
                 CopyAndTouch(AssemblyFile.ItemSpec, outputPath);
-                CopyAndTouch(Path.ChangeExtension(AssemblyFile.ItemSpec, ".pdb"), Path.ChangeExtension(outputPath, ".pdb"));
+                CopyAndTouch(Path.ChangeExtension(AssemblyFile.ItemSpec, ".pdb"), Path.ChangeExtension(outputPath, ".pdb"), false);
 
                 if (!string.IsNullOrEmpty(refOutputPath))
                 {
@@ -49,8 +49,18 @@ namespace Avalonia.Build.Tasks
             return res.Success;
         }
 
-        private static void CopyAndTouch(string source, string destination)
+        private static void CopyAndTouch(string source, string destination, bool shouldExist = true)
         {
+            if (!File.Exists(source))
+            {
+                if (shouldExist)
+                {
+                    throw new FileNotFoundException($"Could not copy file '{source}'. File does not exist.");
+                }
+
+                return;
+            }
+
             File.Copy(source, destination, overwrite: true);
             File.SetLastWriteTimeUtc(destination, DateTime.UtcNow);
         }