Special-case coalescing of SUM in SqlNullability is causing issues

[roji]

In my eternal quest to translate all aggregate functions on the planet (and beyond), I'm adding support for Sum and Average over TimeSpan for PostgreSQL, where that's supported (npgsql/efcore.pg#2339).

For sum, I ran into trouble because of the following code in SqlNullabilityProcessor:

if (sqlFunctionExpression.IsBuiltIn
    && string.Equals(sqlFunctionExpression.Name, "SUM", StringComparison.OrdinalIgnoreCase))
{
    nullable = false;

    return _sqlExpressionFactory.Coalesce(
        sqlFunctionExpression.Update(instance, arguments),
        _sqlExpressionFactory.Constant(0, sqlFunctionExpression.TypeMapping),
        sqlFunctionExpression.TypeMapping);
}

This fails because it attempts to coalesce TimeSpan (non-numeric) to zero.

Replacing the 0 with Activator.CreateInstance(sqlFunctionExpression.Type) makes the problem go away. However, since I'm translating EF.Functions.Sum, the return type of that function is nullable (TimeSpan?), since that's how SQL sum behaves (compare with #26634). So the COALESCE introduced here is unwanted.

It generally seems better to handle this kind of thing in the method translator (but I don't have the context) - is that possible?

/cc @maumar @smitpatel

[roji]

@maumar I've implemented a hacky workaround on my side for this, i.e. I just remove the bad coalesce node in NpgsqlSqlNullabilityProcessor.

This means this is no longer blocking and can be punted out if needed (though I believe it should still be done).