getEditableSites() is empty when only 1 site is defined

[janhenckens]

Description

On a Craft site with only 1 site defined, the Sites section is missing from the user permissions screen, resulting in getEditableSites() returning nothing.

The user clearly has permission to edit the single site since I can give it access to all the sections & content of the site.

I'm using getEditableSites in a new plugin to determine to which site the user should have access, but that doesn't work for a single site install.

Steps to reproduce

1. Create a clean craft install
2. Add a new user and don't it an admin user
3. Check Craft::$app->sites->getEditableSites()

Additional info

• Craft version: 3.0.13.2
• PHP version: 7.2.1
• Database driver & version: MySQL 5.6.38
• Plugins & versions: none