Better translation for COUNT with predicate checking for non-null

We currently translate the following LINQ query:

```c#
_ = ctx.Blogs
.GroupBy(b => b.Id)
.Select(g => new { g.Key, Count = g.Select(x => x.Name).Count(n => n != null) })
.ToList();
```

... to:

```sql
SELECT [b].[Id] AS [Key], COUNT(CASE
WHEN [b].[Name] IS NOT NULL THEN 1
END) AS [Count]
FROM [Blogs] AS [b]
GROUP BY [b].[Id]
```

... when we could just translate to:

```sql
SELECT [b].[Id] AS [Key], COUNT([b].[Name]) AS [Count]
FROM [Blogs] AS [b]
GROUP BY [b].[Id]
```

... since `COUNT(x)` only counts non-null values. This may have a perf impact (index usage).

Related to #26938