Remove DocumentMapperForType

[javanna]

No description provided.

[javanna]

Closing this. It was a good, unfortunately failed, experiment. The conclusion is that I was trying to take a shortcut which almost works but not entirely, which would cause us to have edge case situations where an index with only empty docs indexed has no mappings, which we don't want and it can lead to unexpected situations.

I have incorporated part of the changes that I had made in #72400, but without the removal that I was targeting initially. The approach we should take is to rather eagerly create a DocumentMapper whenever a new index is created. That way we don't have any situation where an index has no mappings, hence there is no need for DocumentMapperForType to exist. This approach has a couple of implications: we need to check all the consumers of MapperService#documentMapper and remove/replace their null checks; we need to have a different way to figure out whether an index is empty or not (may be useful to rewrite queries to match_none when there are no docs that can possibly match). Lastly, we cannot adopt this approach in 7.x, because a type that is not _doc is still accepted hence we can't eagerly create mappings for a given index, as its type may have to change later depending on what the user requests.