make compatible with new SDK versions

[felipao-mx]

tests fails when running on a new dart SDK version. here is the error log using SDK >= 2.18.2

Installing Dart SDK version "latest" from the stable channel (release) on linux-x64
Downloading https://storage.googleapis.com/dart-archive/channels/stable/release/latest/sdk/dartsdk-linux-x64-release.zip...
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed

  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
[10](https://github.com/cuenca-mx/clabe-dart/actions/runs/3277530311/jobs/5394925508#step:3:12)0  180M  100  180M    0     0   213M      0 --:--:-- --:--:-- --:--:--  213M
Pub cache set to: /home/runner/.pub-cache
Successfully installed Dart SDK:
Dart SDK version: 2.18.2 (stable) (Tue Sep 27 13:24:[11](https://github.com/cuenca-mx/clabe-dart/actions/runs/3277530311/jobs/5394925508#step:3:13) 2022 +0200) on "linux_x64"
....
00:00 +0: loading test/clabe_test.dart                                                                                                                                                                 
00:01 +0: loading test/clabe_test.dart                                                                                                                                                                 
00:01 +0 -1: loading test/clabe_test.dart [E]                                                                                                                                                          
  Failed to load "test/clabe_test.dart":
  Unable to spawn isolate: test/clabe_test.dart: Error: A library can't opt out of null safety by default, when using sound null safety.
  Error: Cannot run with sound null safety, because the following dependencies
  don't support null safety:
  
   - package:test_core
   - package:test_api
   - package:test
   - package:clabe_dart
   - package:boolean_selector
   - package:matcher
  
  For solutions, see https://dart.dev/go/unsound-null-safety

00:01 +0 -1: Some tests failed.                                                                                                                                                                        
Error: Process completed with exit code 1.