EXEC in T-SQL ignores CONCAT_NULL_YIELDS_NULL

Question

With any version of Sql Server, I cannot understand why, in the following script, EXEC returns the result of GetDate(). I expected no result.

SET CONCAT_NULL_YIELDS_NULL ON;
DECLARE @sql_select  nvarchar(150) = 'SELECT GetDate()';
DECLARE @sql_select2 nvarchar(150) = NULL;
SELECT  @sql_select + @sql_select2
EXEC(@sql_select + @sql_select2)

Answer 1

I think, inside exec, NULL is converted into empty string. Here is an easier example

DECLARE @sql_select2 varchar(200) ;

exec('select 1' + @sql_select2) 

returns 1 though it is concatenated with a NULL value.

But when we do the concatenation outside it works as expected.

DECLARE @sql_select NVARCHAR(150) = 'SELECT GetDate()';
DECLARE @sql_select2 NVARCHAR(150) = NULL;
DECLARE @sql VARCHAR(500)

SET @sql = @sql_select + @sql_select2

EXEC(@sql) 

returns nothing