Steps to reproduce

1. Structure
2. Pick a generated column, Change
3. Uncheck "NULL", and default value to none

Expected behaviour

Should change default to "None" instead of NULL

Actual behaviour

Default value stays as NULL

Server configuration

Operating system: Windows 10 Pro

Web server:
Apache 2.4.23

Database:
MySQL 5.7.14

PHP version:
5.6.25

phpMyAdmin version:
4.6.5.2

Client configuration

Browser:
Chrome

Operating system:
Windows

I haven't tried, but I imagine it probably happens in VIRTUAL GENERATE COLUMNS as well.

Here is the SQL query executed though I checked NOT NULL.
ALTER TABLE storage_file CHANGE file_url file_url VARCHAR(510) AS (replace(file_url_template,'{$path}',file_url_path)) STORED;

Thanks!