Change license key form field from required to option

Problem/Motivation

On the General settings from the License key is a required form field. This is a problem where the licence key is provided by a configuration override, like using environment variables in settings.php, or using the Key module.

In those situations it is preferable to leave the licence key form field blank, as right now it is required to provide a value, so this ends up being a dummy value.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes