fix prelive check: prepay_enabled IS NOT TRUE not IS NULL

Migration 0015 narrowed the original 0000 condition from
(IS NULL OR = false) to IS NULL only, breaking the prelive
transition when prepay_enabled is explicitly set to false.

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

Author: chatch

sqitch/deploy/0021_corporate_body_customer_status.sql

@@ -104,7 +104,7 @@ BEGIN
             WHERE ca.customer = new_customer_row.id
             AND ca.role = 'occupier'
             AND a.type = 'supply'
-            AND (m.prepay_enabled IS NULL)
+            AND (m.prepay_enabled IS NOT TRUE)
         ) INTO has_unprepared_supply_meter;
     END IF;