The schema checker's change item class for PostgreSQL doesn't understand "varchar", it need "character vayring".
In PostgreSQL you change the "NOT NULL" contrsaint for a column or the default value with a separate statement, or at least our schema checker expects it like this for PostgreSQL.
Testing Instructions
Can be merged by review.
Expected result
Update from current 3.10-dev suceeds.
Actual result
ERROR: syntax error at or near "VARCHAR" at character 52 STATEMENT: ALTER TABLE "j3ux0_history" ALTER COLUMN "item_id" VARCHAR(50) NOT NULL DEFAULT "";
Thanks!