[fix] check if academic_year exists

[saurabh6790]

Traceback (most recent call last):
  File "/home/frappe/benches/bench-2018-03-27/apps/frappe/frappe/app.py", line 62, in application
    response = frappe.handler.handle()
  File "/home/frappe/benches/bench-2018-03-27/apps/frappe/frappe/handler.py", line 22, in handle
    data = execute_cmd(cmd)
  File "/home/frappe/benches/bench-2018-03-27/apps/frappe/frappe/handler.py", line 53, in execute_cmd
    return frappe.call(method, **frappe.form_dict)
  File "/home/frappe/benches/bench-2018-03-27/apps/frappe/frappe/__init__.py", line 939, in call
    return fn(*args, **newargs)
TypeError: get_students() takes at least 2 arguments (3 given)