Update database schema to not allow null previous_content

pkg/database/migrations/000005_remove_null_from_lists.down.sql

@@ -0,0 +1,6 @@
+BEGIN;
+    ALTER TABLE messages
+        ALTER COLUMN previous_content DROP NOT NULL;
+    ALTER TABLE messages
+        ALTER COLUMN previous_content DROP DEFAULT;
+COMMIT;

pkg/database/migrations/000005_remove_null_from_lists.up.sql

@@ -0,0 +1,7 @@
+BEGIN;
+    UPDATE messages SET previous_content = array[]::varchar[];
+    ALTER TABLE messages
+        ALTER COLUMN previous_content SET NOT NULL;
+    ALTER TABLE messages
+        ALTER COLUMN prevous_content SET DEFAULT array[]::varchar[]
+COMMIT;