PHP warning on upgrade from Drupal 7 in system_get_date_formats()

[jenlampton]

Description of the bug

I'm seeing the following PHP warning when I upgrade sites from Drupal 7:

Warning: Invalid argument supplied for foreach() in system_get_date_formats() (line 3911 of system.module 

I know this isn't an issue because if you push past it and complete the upgrade everything then works fine, but it would be nice if we could avoid any kind of scary warnings during the upgrade process, which can be stressful enough :)

Steps To Reproduce

To reproduce the behavior:

1. Load in Drupal 7 database
2. Visit core/update.php
3. Error appears on this page

Additional information

Add any other information that could help, such as:

• Backdrop CMS version: 1.20.1
• Web server and its version:
• Operating System and its version:
• Browser(s) and their versions:

[bugfolder]

Tested this on a Drupal upgrade that had been showing the warning; this fixed the warning. Code looks good. LGTM.

[quicksketch]

Thanks, merged backdrop/backdrop#3786 into 1.x and 1.20.x!