codegen: handle nullable foreign keys using 'ifnull' and add a test case for them

ops/gas_init_test.sh

@@ -31,9 +31,12 @@ cd $test_project
 
 # Create a new table in the schema
 cat >> pkg/db/schema.sql <<EOF
+create table item_flavor (rowid integer primary key, name text not null) strict;
+
 create table items (
     rowid integer primary key,
-    description text not null default ''
+    description text not null default '',
+    flavor integer references item_flavor(rowid)
 ) strict;
 EOF