dbt-utils generate_surrogate_key macro fails for versions 1.4+ when only one field is supplied

[amychen1776]

When an user uses the macro with only one field supplied on the adapter versions 1.4 and above, it fails with the error message "The concat function requires 2 to 254 arguments". This works on dbt 1.3 and fails after 1.4. I suspect it's related to the migration in 1.4 from depending on the dbt-sqlserver adapter to dbt-fabric.

What I expect to happen is when a user has set surrogate_key_treat_nulls_as_empty_strings to true, the adapter will respect that.

https://github.com/dbt-labs/dbt-utils/blob/1.2.0/macros/sql/generate_surrogate_key.sql