User A company stores data about its employees in two tables, employees and positions, with the following structure: create table employees $($ id integer not null unique, name varchar$(38)$ not null, age integer not null, salary integer not null create table positions $($ id integer not null unique, department$\_$name varchar$(30)$ not null, employee$\_$id integer not null unique, position$\_$name varchar$(38)$ not null $)$; Each record in the table employees represents a single employee and consists of the following data: id$,$ name, age and salary. Each record in the table positions represents the position description of a single employee and consists of the following data: id$,$ department$\_$name, employee$\_$id and position$\_$name. Note: employee$\_$id represents id of employees from previously mentioned employees table. Write an SQL query that calculates the average salary of all employees who are working in a Software Engineer position and are at most $30$ years old. If there are no such employees then the answer should be NULL