Handle NULL for data blob in User source plugin

Problem/Motivation

Getting this error while working on PHP8.1 compatibility in Commerce Migrate. See #3270534: Test failures with PHP 8.1.

1) Drupal\Tests\commerce_migrate_ubercart\Functional\uc7\MigrateUpgradeUbercart7Test::testMigrateUpgradeExecute
Exception: Deprecated function: unserialize(): Passing null to parameter #1 ($data) of type string is deprecated
Drupal\user\Plugin\migrate\source\d7\User->prepareRow()() (Line: 79)

I brought a plain D7 site, 7.90-dev, and created a user and then looked at the users table. And sure enough, there is a NULL. The migrate test fixtures do not have a entry in the users table where data is NULL.

MariaDB [db]> select uid,name,mail,data from users;
+-----+------+-------------------+------+
| uid | name | mail              | data |
+-----+------+-------------------+------+
|   0 |      |                   | NULL |
|   1 | x    | admin@example.com | b:0; |
|   2 | test | test@example.com  | NULL |
+-----+------+-------------------+------+
3 rows in set (0.00 sec)

Steps to reproduce

Proposed resolution

Make sure the unserialize is not sent a NULL.

Remaining tasks

Patch, review, commit.

User interface changes

N/A

API changes

N/A

Data model changes

N/A

Release notes snippet

N/A