Recency test to fail when no recent records are found

[marzaccaro]

resolves #1041

Problem

The dbt_utils.recency test incorrectly passes when a where clause filters
out all records.

Current behavior: When a where clause results in no matching records,
most_recent becomes NULL. The test only checks where most_recent < {{ threshold }}, but since NULL < threshold evaluates to NULL (not true), no
rows are returned and the test incorrectly passes.

Expected behavior: The test should fail when no recent data exists (even
when filtered by a where clause), indicating missing recent data.

Solution

Modified the recency macro in macros/generic_tests/recency.sql to include
or most_recent is null in the WHERE clause. This ensures that when a where
clause filters out all records (causing most_recent to be null), the test
correctly fails by returning the null row.

Changes made:

• Updated the WHERE condition from where most_recent < {{ threshold }} to
  where most_recent < {{ threshold }} or most_recent is null
• Added integration test recency_with_where_filter that reproduces the
  original bug and verifies the fix
• Test uses data with old timestamps (10-15 days) and a where clause that
  filters to non-existent records

Testing approach:

• Before fix: Test incorrectly passes (returns 0 rows)
• After fix: Test correctly fails (returns 1 row with null most_recent)
• Verified existing recency tests continue to work as expected

Checklist

• This code is associated with an
  issue which has been
  triaged and [accepted for development](https://docs.getdbt.com/docs/contributin
  g/oss-expectations#pull-requests).
• I have read the contributing
  guide and
  understand what's expected of me
• I have run this code in development and it appears to resolve the stated
  issue
• This PR includes tests, or tests are not required/relevant for this PR
• I have updated the README.md (if applicable)