fix(stg_ofsted_inspections): correctly filter NULL string inspection dates

The string 'NULL' is not SQL NULL, so the WHERE in the renamed CTE
passed those rows through. Filter on the raw value using nullif in the
CTE and on the computed date in the outer SELECT.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

pipeline/transform/models/staging/stg_ofsted_inspections.sql

@@ -36,7 +36,8 @@ renamed as (
         report_url
     from source
     where urn is not null
-      and inspection_date is not null
+      and nullif(trim(inspection_date), 'NULL') is not null
 )
 
 select * from renamed
+where inspection_date is not null