Install profiles can now be uninstalled

The install profile can now be uninstalled if the site does not depend on any module or theme in the profile directory. This is validated by InstallProfileUninstallValidator.

This means that it becomes possible for Drupal sites to be installed without using an install profile.

Before

\Drupal::installProfile() can return NULL (not yet installed) or a string value (the machine name of the install profile).

core.extension:

module: {}
theme: {}
profile: ''

… and the profile key MUST exist.

After

\Drupal::installProfile() can return FALSE (no install profile), NULL (not yet installed) or a string value (the machine name of the install profile).

Default core.extension:

module: {}
theme: {}
profile: null

… and the profile key MAY exist: if it's removed, that corresponds to \Drupal::installProfile() === FALSE i.e. no install profile.

So, from a developer's perspective, there are only two things to be aware of:

1. \Drupal::installProfile() might return FALSE, meaning there is no install profile anymore.
2. The core.extension:profile config key won't exist if the profile is gone.