Review MySQL default null and nullable column definition

[Jeckerson]

Hello!

• Type: bug fix | code quality
• Link to issue: Fix mysql column numeric default #14888

In raising this pull request, I confirm the following:

• I have read and understood the Contributing Guidelines
• I have checked that another pull request for this purpose does not exist
• I wrote some tests for this PR
• I have updated the relevant CHANGELOG
• I have created a PR for the documentation about this change

Current PR fixes previous PRs (#14888) behavior, when it wasn't possible to specify NULL in column definition, as some of types requires it, if column need to be nullable.
For example:
This is NOT NULL

updated_at TIMESTAMP

This is NULL, with default NULL

updated_at TIMESTAMP NULL

This is also NULL, with default NULL

updated_at TIMESTAMP NULL DEFAULT NULL

Two last examples was not possible to generate before this PR.

---

Also, moved NULL/NOT NULL before DEFAULT.

Before:

updated_at` TIMESTAMP DEFAULT NULL NULL

After

updated_at` TIMESTAMP NULL DEFAULT NULL

Previous branch - #14973

Thanks

[sergeyklay]

I have some questions, probably due to the fact that the changes are not quite clear to me. There is no issue link, nor PR description

[Jeckerson]

Added link of previous PR (#14888) in the description.

[Jeckerson]

Updated explanation into description.

[sergeyklay]

Looks good to me

[sergeyklay]

Thank you