Accept to load an application without a specific icon defined

This fixes the following issue:

PHP Fatal error:
Typed property Waystone\Workspaces\Engines\Apps\ApplicationConfiguration::$icon
must not be accessed before initialization
in /var/51-wwwroot/obsidian/workspaces/src/controllers/header.php:61

Differential Revision: https://devcentral.nasqueron.org/D3830