Make table and column comments non-nullable strings

[morozov]

Q	A
Type	improvement
BC Break	yes

1. The $comment arguments were marked as nullable during the scalar types enforcement in the codebase (Enforced parameter and return value types in Platform classes #3562, Added type hints to the Column class #3511) because, despite the fact they were annotated as string in phpDoc, the values might contain NULL at runtime.
2. In practice a NULL comment and an empty comment are indistinguishable (there either is a comment or there's no).
3. Although the underlying platforms support nullable comments (except for SQLite), they are impossible to implement for commented types that store their type on the comment.
4. The codebase currently contains expressions like (string) $comment or if ($comment === null || $comment === '') which illustrates that there's no practical difference between them at the abstraction level.
5. The removed functional tests were removed in favor of the newly introduced ColumnCommentTest that is cross-platform and more focused on the API rather than on implementation details.

[greg0ire]

Phpstan is ok with mixed[][], but Psalm isn't.

Suggested change

	* @return mixed[][]
	* @return iterable<mixed[]>

[morozov]

As far as I understand, [] is a list/collection. It doesn't imply a specific implementation. It looks like Psalm thinks differently (cannot find a reference).

I'm fine to adjust but I'd rather do it for the entire codebase and have the rule enforced on CI. E.g. by adding a dependency on Psalm and setting the lowest possible level. Otherwise, it will become a matter of personal preference rather than a standard.

$ git grep 'mixed[][]' tests | wc -l
96

$ git grep 'iterable<mixed\[\]>' tests | wc -l
0

[greg0ire]

Ok let's not make this a blocker

[greg0ire]

Suggested change

	* @return mixed[][]
	* @return iterable<mixed[]>