Format _empty.dart (flutter#57144)

When `dartfmt` formatted `_empty.dart` it formatted it with a trailing newline, which then makes the license script angry. Rather then treating `_empty.dart` special, this just makes it a regular source file with header and everything.

Author: goderbauer

ci/licenses_golden/licenses_flutter

@@ -45301,6 +45301,7 @@ ORIGIN: ../../../flutter/shell/profiling/sampling_profiler.h + ../../../flutter/
 ORIGIN: ../../../flutter/shell/version/version.cc + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/version/version.h + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/vmservice/empty.dart + ../../../flutter/LICENSE
+ORIGIN: ../../../flutter/sky/packages/sky_engine/lib/_empty.dart + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/third_party/accessibility/base/color_utils.h + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/third_party/accessibility/base/compiler_specific.h + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/third_party/accessibility/base/container_utils.h + ../../../flutter/LICENSE
@@ -48266,6 +48267,7 @@ FILE: ../../../flutter/shell/profiling/sampling_profiler.h
 FILE: ../../../flutter/shell/version/version.cc
 FILE: ../../../flutter/shell/version/version.h
 FILE: ../../../flutter/shell/vmservice/empty.dart
+FILE: ../../../flutter/sky/packages/sky_engine/lib/_empty.dart
 FILE: ../../../flutter/third_party/accessibility/base/color_utils.h
 FILE: ../../../flutter/third_party/accessibility/base/compiler_specific.h
 FILE: ../../../flutter/third_party/accessibility/base/container_utils.h

sky/packages/sky_engine/lib/_empty.dart

@@ -0,0 +1,3 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.