🩹 Fix loop_vars when there is no variable

macros/loop_vars.sql

@@ -22,7 +22,7 @@
 {%- if sql_query -%}
 {{ sql_query | join('union all') }}
 {%- else -%}
-select '' as var_name, null as var_value
-where var_value is not null
+select '' as var_name, '' as var_value
+limit 0
 {%- endif -%}
 {% endmacro %}