[WIP] Concating null with not null string resulting in null

[Muppets]

DO NOT COMMIT THIS - work in progress.

Looking for some feedback on WIP for issue #3836.

My attack plan was to include a Coalesce function call with all string concatenations. This does produce somewhat ugly SQL for a ton of common scenarios. I changed the check to only wrap references to nullable columns in Coalesce but this will miss a ton of different possible outcomes.

Looking for some guidance on what you fancy doing here, uglify everything or only cover some really specific examples?

I've not updated all the unit tests to match this change yet so there are broken tests.

[smitpatel]

• This work has conflict with New Query Pipeline #14455 . Precisely a lot of this code is going away.
• string.Concat comes as ExpressionType.Add, in new pipeline we just decided to move it to method call translation.

[smitpatel]

As for the fix, the code would change altogether due to SQL tree being different in new pipeline. We should be able to apply this optimization more smartly once we have null expansion metadata, then for the very specific case of string concat when argument could be nullable, we could apply coalesce.

[Muppets]

Sweet, sounds like the new query pipeline will make this a ton easier. I'll close this out and we can revisit the issue once that is merged.