rework-onboarding (#7)

Onboarding completato, ora super rapido e top

Reviewed-on: http://catelliub.zapto.org:3000/brontomark/flux/pulls/7
Co-authored-by: Mark M2 Macbook <marco@catelli.it>
Co-committed-by: Mark M2 Macbook <marco@catelli.it>

lib/core/utils/validators.dart

@@ -0,0 +1,6 @@
+String? notEmptyValidator(String? value) {
+  if (value == null || value.trim().isEmpty) {
+    return 'Campo obbligatorio';
+  }
+  return null;
+}