Check for FileSystemEntityType.notFound (#80)

The Dart breaking change <https://github.com/dart-lang/sdk/issues/40706>
will change the timestamps on the notFound object from null to the epoch
timestamp.
diff --git a/lib/src/stat.dart b/lib/src/stat.dart
index 6430d0b..fb157a5 100644
--- a/lib/src/stat.dart
+++ b/lib/src/stat.dart
@@ -28,5 +28,6 @@
   }
 
   final stat = await FileStat.stat(path);
+  if (stat.type == FileSystemEntityType.notFound) return null;
   return stat.modified;
 }