Add NotBlankAfterInstall constraint and use it for system.date:timezone.default

Problem/Motivation

We made system.date:timezone.default nullable in #3437325: Add validation constraints to system.date but a NULL is not a valid value once Drupal has been installed. We set the default timezone in the installer.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet