Fix: Knex cannot set a default value for TEXT field (MariaDB)

[louislam]

Description

Read more: #5048 (comment)
knex/knex#2649

As this issue was reported in 2018, I don't think it will fixed in soon, so we have to do it in Uptime Kuma.

Type of change

Please delete any options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

Checklist

• My code follows the style guidelines of this project
• I ran ESLint and other linters for modified files
• I have performed a self-review of my own code and tested it
• I have commented my code, particularly in hard-to-understand areas (including JSDoc for methods)
• My changes generates no new warnings
• My code needed automated testing. I have added them (this is optional task)