nnbd_migration: Add missing return statement to nullably-typed function

Bug: https://github.com/dart-lang/sdk/issues/46656
Change-Id: Ifbc498763fdc38440f12f394a5981c4d70fd83bf
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/222380
Reviewed-by: Paul Berry <paulberry@google.com>
Commit-Queue: Samuel Rawlins <srawlins@google.com>

pkg/nnbd_migration/lib/migration_cli.dart

@@ -358,6 +358,7 @@ class MigrationCli {
           logger: isVerbose! ? loggerFactory(true) : null);
     } on Object catch (exception) {
       handleArgParsingException(exception);
+      return null;
     }
   }