Removal of compilation warning

This PR fixes the following issue: ``` /C:/src/flutter/.pub-cache/hosted/pub.dartlang.org/screenshot-1.2.0/lib/screenshot.dart:45:31: Warning: Operand of null-aware operation '??' has type 'Duration' which excludes null. - 'Duration' is from 'dart:core'. return new Future.delayed(delay ?? Duration(milliseconds: 20), () async { ^ ```

Removal of compilation warning
2b4f67e7 · Yaniv Shaked · GitHub · 8eaef5f3 · 2b4f67e7
--- a/lib/screenshot.dart
+++ b/lib/screenshot.dart
@@ -42,7 +42,7 @@ class ScreenshotController {
    Duration delay = const Duration(milliseconds: 20),
  }) {
    //Delay is required. See Issue https://github.com/flutter/flutter/issues/22308
-    return new Future.delayed(delay ?? Duration(milliseconds: 20), () async {
+    return new Future.delayed(delay, () async {
      try {
        ui.Image image = await captureAsUiImage(
          delay: Duration.zero,