Fix to #18916 - Query: regression in EFCore 3.1 Preview 2 and Preview3 for query with conditional expression whose element is a comparison containing null-value parameter

[maumar]

Problem was that now we are peeking into parameter values and simplifying the queries. As a result comparisons of non-nullable expressions with null-value parameter are optimized out to a simple constant false. Sql server expects search condition in some places (e.g. WHERE, HAVING or WHEN parts of CASE statements).
Before we look into parameter values we run SearchConditionConvertingExpressionVisitor which fixes those cases. However, this visitor is currently sql server specific, and the part of the pipeline that looks into parameter values is not provider specific, so we can't re-run it.

Fix is to manually convert constants encountered in those areas into conditions. We used to do it already for WHERE and HAVING but we missed WHEN.

Resolves #18916

[smitpatel]

This has build check failures. Can you investigate?

[maumar]

Customer Impact
Query failure (invalid sql) when using conditional expression when result expression is a comparison with null-valued parameter.

How found
Customer-reported (#18916)

Test coverage
We did not have tests when all necessary conditions were met.

Regression?
Yes, from 3.0.

Risk
Risk is very small - fix is very targeted and we already apply the same logic in two other places. Just missed the 3rd case where it was needed.

Workaround:
Workaround is to rewrite the query so that comparison with null-value parameter is not happening in the result of case expression. Workaround is relatively simple to implement but not intuitive. Alternatively, 2 separate queries can be written, one for prm2 being null and one when its not null.

original query:

var prm1 = true;
var prm2 = default(string);
var query = customers.Where(c => c.IsVip == true ? true : c.Name == prm2);

workaround:

var prm1 = true;
var prm2 = default(string);
var query = customers.Where(c => c.IsVip == true ? true : prm2 == null ? c.Name == null : c.Name == prm2);

[maumar]

@Pilchie @ajcvickers

[ajcvickers]

@Pilchie @wtgodbe @dougbu Can this now be merged for 3.1.1?