fix(recency): handle empty tables returning NULL (dbt-labs#1065)

When a table is empty, MAX(field) returns NULL. The comparison
NULL < threshold evaluates to NULL (not TRUE), causing the WHERE
clause to filter out the row and incorrectly pass the test.

This fix adds 'OR most_recent IS NULL' to properly fail the test
when a table has no data within the recency window, including when
the table is completely empty.

Author: dinaatmiro

macros/generic_tests/recency.sql

@@ -37,5 +37,6 @@ select
 
 from recency
 where most_recent < {{ threshold }}
+   or most_recent is null
 
 {% endmacro %}