[release/10.0] Correct COALESCE logic for DefaultIfEmpty (#37233)

[roji]

Backports #37233
Closes #37178

Description

10.0 contained various bug fixes to EF's DefaultIfEmpty() LINQ operator translation. One of these bugfixes required making changes to how COALESCE() is added for non-nullable value types, where we need to return the CLR default (e.g. zero for int) rather than NULL. Unfortunately, to determine whether the return value is a non-nullable value type, an incorrect type was consulted in the SQL tree, which indicates a non-nullable value type even when the returned value is in fact a nullable value type.

Customer impact

In cases where DefaultIfEmpty() is used over a nullable value type, NULL is not returned as expected, but rather than CLR default.

var items = await context.Products
    .Select(x => x.NullableInt)
    .DefaultIfEmpty()
    .ToListAsync();

This is a data corruption bug, indicating that there's data in the database (zero), where in fact there's no data (and NULL should be returned).

How found

Customer reported on 10.0.0

Regression

Yes, from 9.0.

Testing

Added.

Risk

Low, very targeted fix (most code in this PR only flows information in, actual behavioral change is a one-liner). Quirk added.

[artl93]

Customer reported, regression, data corruption.