Fix migration for cases with empty strings

Contributes to https://gitlab.com/gitlab-org/gitlab/-/issues/349313

Skip empty strings for auth token encryption migration

Changelog: fixed

db/migrate/20211126115449_encrypt_static_objects_external_storage_auth_token.rb

@@ -13,6 +13,8 @@ class EncryptStaticObjectsExternalStorageAuthToken < Gitlab::Database::Migration
     ApplicationSetting.reset_column_information
 
     ApplicationSetting.encrypted_token_is_null.plaintext_token_is_not_null.find_each do |application_setting|
+      next if application_setting.static_objects_external_storage_auth_token.empty?
+
       token_encrypted = Gitlab::CryptoHelper.aes256_gcm_encrypt(application_setting.static_objects_external_storage_auth_token)
       application_setting.update!(static_objects_external_storage_auth_token_encrypted: token_encrypted)
     end

spec/migrations/20211126115449_encrypt_static_objects_external_storage_auth_token_spec.rb

@@ -53,4 +53,26 @@ RSpec.describe EncryptStaticObjectsExternalStorageAuthToken, :migration do
       end
     end
   end
+
+  context 'when static_objects_external_storage_auth_token is empty string' do
+    it 'does not break' do
+      settings = application_settings.create!
+      settings.update_column(:static_objects_external_storage_auth_token, '')
+
+      reversible_migration do |migration|
+        migration.before -> {
+          settings = application_settings.first
+
+          expect(settings.static_objects_external_storage_auth_token).to eq('')
+          expect(settings.static_objects_external_storage_auth_token_encrypted).to be_nil
+        }
+        migration.after -> {
+          settings = application_settings.first
+
+          expect(settings.static_objects_external_storage_auth_token).to eq('')
+          expect(settings.static_objects_external_storage_auth_token_encrypted).to be_nil
+        }
+      end
+    end
+  end
 end