Added NOT NULL constraint to db_name.

sql/patch_46_47_c.sql

@@ -6,7 +6,7 @@
 # Make db_release column of external_db VARCHAR(255)
 
 ALTER TABLE external_db CHANGE COLUMN db_release db_release VARCHAR(255);
-ALTER TABLE external_db CHANGE COLUMN db_name db_name VARCHAR(28);
+ALTER TABLE external_db CHANGE COLUMN db_name db_name VARCHAR(28) NOT NULL;
 
 # patch identifier
 INSERT INTO meta (meta_key, meta_value) VALUES ('patch', 'patch_46_47_c.sql|extend_db_release');