Fix nullable colums for subqueries in left and full joins

[richardbizik]

There is an issue when performing join on subquery. Fields that should be nullable are not because node is processed as ordinary select node. This pull request fixes the issue.

[kyleconroy]

Sorry, the current approach taken in this PR isn't going to work. The sample query added to the left join test is helpful to identify the problem.

[richardbizik]

I made some changes. Would this approach work better?

[sbres]

I've tried to run the sql query but I seem unable to make it work as is.

Could you check it ?
https://www.db-fiddle.com/f/hXNdApf4mV3Qi1jEgu2egZ/3

[richardbizik]

here is fixed fiddle https://www.db-fiddle.com/f/f8yeYgvggnmw1t7p3M5YE6/1
I am going to update code as well.

[kyleconroy]

Is this trying to fix the same problem as #1486? If not, I'd like to merge that first before we continue working on this change.

You have some merge conflicts here. Can you rebase?

[sbres]

I've closed #1486 as #1491 applies the same fix.
But they do not fix what this PR is trying to achieve

[richardbizik]

As @sbres pointed out they are fixing a different issue. This PR is trying to solve an issue with nested select statements where an information is lost about the parent being join that should produce nullable results.

[kdubovikov]

I think this fix is what we need to solve #1334