Fix comma condition for alter_relation_add_remove_columns

[jtcohen6]

Noticed this while testing on_schema_change with different incremental strategies to chase down an unrelated bug. If we have remove_columns but no add_columns, we're going to add a comma where we don't need one:

dbt-core/core/dbt/include/global_project/macros/adapters/common.sql

Line 334 in ff109e1

{% endfor %}{{ ',' if remove_columns | length > 0 }}

alter table "jerco"."dbt_jcohen"."my_incremental_model"
       
            ,
            
            
                drop column another_new_column

This should be a super quick fix.