Fixed #11847 404 error during Import if a checkout type of location is specified and no checkout location is passed

[inietov]

Description

Adds a condition to see if a checkout location is passed from the CSV to the importer. I just ignored the field if the CSV have the field empty, because looking at the code, rtd_location is used as base checked-out to location, and then overwriten for the one passed in checkout_location, so I thought the best approach was to not look for a location if no location is specified.

Fixes #11847

Type of change

Please delete options that are not relevant.

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

How Has This Been Tested?

Test Configuration:

• PHP version: 8.1.10
• MySQL version: 8.0.23
• Webserver version: nginx/1.19.8
• OS version: Debian 10

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