SQL Server 2017, by default, will consider null values in CONCAT() function as empty strings and return a non-null value. But, it also offers the option of changing this setting by using CONCAT_NULL_YIELDS_NULL setting. They do warn, though, that this option will be ON in a future version of SQL Server: https://docs.microsoft.com/en-us/sql/t-sql/statements/set-concat-null-yields-null-transact-sql?view=sql-server-ver15. The preview docs for SQL Server 2019 does still have the option default as OFF (so null is considered empty string).

PostgreSQL will also "ignore" null values and treat them as empty strings: https://www.postgresql.org/docs/11/functions-string.html