Fixes #8837 Importer issue when running from CLI.

[inietov]

Description

If the importer is launched from the CLI, no user was passed to the CheckoutableCheckedOut.php event.

In the Models/Asset.php@checkout() function I saw an unused variable called $admin that I thought I can use for this because ObjectImportCommand.php already set an user_id that I pass to that unused variable. Then I search for that user and pass it to the event, so it doesn't crash.

See the changes for more info.

Fixes #8837

• Bug fix (non-breaking change which fixes an issue)

Test Configuration:

• PHP version: 7.4.14
• MySQL version: mariadb Ver 15.1 Distrib 10.4.17-MariaDB
• Webserver version: nginx/1.18.0
• OS version: Fedora 33

Checklist:

• I have read the Contributing documentation available here: https://snipe-it.readme.io/docs/contributing-overview
• I have formatted this PR according to the project guidelines: https://snipe-it.readme.io/docs/contributing-overview#pull-request-guidelines
• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[uberbrady]

This looks great! We'll probably want to do a few test passes to make sure - but I think this approach makes great sense. Excellent!

[inietov]

I test it with the sample CSV in the docs, and also modified it a little. But I agree that some users CSVs can be different...