Make start and end date as mandatory

[shreyashah115]

Error due to calendar view if start and end date are blank.

Traceback (most recent call last):
  File "/home/frappe/benches/bench-2018-07-19/apps/frappe/frappe/app.py", line 62, in application
    response = frappe.handler.handle()
  File "/home/frappe/benches/bench-2018-07-19/apps/frappe/frappe/handler.py", line 22, in handle
    data = execute_cmd(cmd)
  File "/home/frappe/benches/bench-2018-07-19/apps/frappe/frappe/handler.py", line 53, in execute_cmd
    return frappe.call(method, **frappe.form_dict)
  File "/home/frappe/benches/bench-2018-07-19/apps/frappe/frappe/__init__.py", line 939, in call
    return fn(*args, **newargs)
  File "/home/frappe/benches/bench-2018-07-19/apps/frappe/frappe/desk/calendar.py", line 46, in get_events
    return frappe.get_list(doctype, fields=fields, filters=filters)
  File "/home/frappe/benches/bench-2018-07-19/apps/frappe/frappe/__init__.py", line 1150, in get_list
    return frappe.model.db_query.DatabaseQuery(doctype).execute(None, *args, **kwargs)
  File "/home/frappe/benches/bench-2018-07-19/apps/frappe/frappe/model/db_query.py", line 88, in execute
    result = self.build_and_run()
  File "/home/frappe/benches/bench-2018-07-19/apps/frappe/frappe/model/db_query.py", line 100, in build_and_run
    args = self.prepare_args()
  File "/home/frappe/benches/bench-2018-07-19/apps/frappe/frappe/model/db_query.py", line 116, in prepare_args
    self.sanitize_fields()
  File "/home/frappe/benches/bench-2018-07-19/apps/frappe/frappe/model/db_query.py", line 204, in sanitize_fields
    if sub_query_regex.match(field):
TypeError: expected string or buffer