orm: fix null strings (#8497)

vlib/orm/orm_test.v

@@ -29,6 +29,7 @@ fn test_orm_sqlite() {
 	db.exec("insert into User (name, age) values ('Sam', 29)")
 	db.exec("insert into User (name, age) values ('Peter', 31)")
 	db.exec("insert into User (name, age, is_customer) values ('Kate', 30, 1)")
+
 	nr_all_users := sql db {
 		select count from User
 	}
@@ -213,6 +214,12 @@ fn test_orm_sqlite() {
 		select from User order by age desc limit 1
 	}
 	assert updated_oldest.age == 31
+
+	db.exec('insert into User (name, age) values (NULL, 31)')
+	null_user := sql db {
+		select from User where id == 5
+	}
+	assert null_user.name == ''
 }
 
 fn test_orm_pg() {

vlib/v/gen/sql.v

@@ -226,7 +226,11 @@ fn (mut g Gen) sql_select_expr(node ast.SqlExpr) {
 			mut func := 'sqlite3_column_int'
 			if field.typ == table.string_type {
 				func = 'sqlite3_column_text'
-				g.writeln('${tmp}.$field.name = tos_clone(${func}($g.sql_stmt_name, $i));')
+				string_data := g.new_tmp_var()
+				g.writeln('byteptr $string_data = ${func}($g.sql_stmt_name, $i);')
+				g.writeln('if ($string_data != NULL) {')
+				g.writeln('\t${tmp}.$field.name = tos_clone($string_data);')
+				g.writeln('}')
 			} else {
 				g.writeln('${tmp}.$field.name = ${func}($g.sql_stmt_name, $i);')
 			}