Fix nullable entity comparison with null

[bahusoid]

Fixes #2611

[hazzik]

Do I understand correctly that the correct SQL would be like this?

select * from Parent p left join OneToOne  o on p.Id = o.Id where o.Id is null

[bahusoid]

Yes. Left join is needed instead of implicit

[hazzik]

I made it working. Need to resolve TODO. Feel free to drop the commit if you don't like it.

[bahusoid]

I made it working.

I made similar changes in #2612. Only then I tried to make it more general fix not bound to is null comparison. Yeah I also realized that merging both PRs might be a good idea..