Problem/Motivation
\Drupal\Core\Theme\ThemeNegotiator::determineActiveTheme() only checks for NULL so \Drupal\user\Theme\AdminNegotiator::determineActiveTheme() should return NULL when the admin theme is empty — i.e. the admin theme should be the default theme ie. should fallback to \Drupal\Core\Theme\DefaultNegotiator
Proposed resolution
Detect empty string and return NULL instead.
Atm you can also get "0" as a value but that's being taken care of in #2587119: Form sets system.theme:admin to '0' breaking Quick Edit and making no sense
Remaining tasks
User interface changes
None
API changes
None
Data model changes
None
Release notes snippet
N/A