[License] Enable prefer_void_to_null lint

engine/src/flutter/tools/licenses/analysis_options.yaml

@@ -54,6 +54,7 @@ linter:
     - prefer_final_locals
     - prefer_is_not_empty
     - prefer_single_quotes
+    - prefer_void_to_null
     - slash_for_doc_comments
     - sort_constructors_first
     - sort_unnamed_constructors_first

engine/src/flutter/tools/licenses/lib/main.dart

@@ -2260,7 +2260,7 @@ Future<void> _collectLicensesForComponent(_RepositoryDirectory componentRoot, {
 
 // MAIN
 
-Future<Null> main(List<String> arguments) async {
+Future<void> main(List<String> arguments) async {
   final ArgParser parser = ArgParser()
     ..addOption('src', help: 'The root of the engine source')
     ..addOption('out', help: 'The directory where output is written')