[fixes] minor fix for between operator filter

[mbauskar]

steps to recreate the issue

1. In filters select the field with fieldtype date or datetime
2. Select the operator as Between and set the value as Null

Traceback (most recent call last):
  File "/home/frappe/benches/bench-2017-03-30/apps/frappe/frappe/app.py", line 55, in application
    response = frappe.handler.handle()
  File "/home/frappe/benches/bench-2017-03-30/apps/frappe/frappe/handler.py", line 19, in handle
    execute_cmd(cmd)
  File "/home/frappe/benches/bench-2017-03-30/apps/frappe/frappe/handler.py", line 40, in execute_cmd
    ret = frappe.call(method, **frappe.form_dict)
  File "/home/frappe/benches/bench-2017-03-30/apps/frappe/frappe/__init__.py", line 901, in call
    return fn(*args, **newargs)
  File "/home/frappe/benches/bench-2017-03-30/apps/frappe/frappe/desk/reportview.py", line 17, in get
    data = compress(execute(**args), args = args)
  File "/home/frappe/benches/bench-2017-03-30/apps/frappe/frappe/desk/reportview.py", line 22, in execute
    return DatabaseQuery(doctype).execute(*args, **kwargs)
  File "/home/frappe/benches/bench-2017-03-30/apps/frappe/frappe/model/db_query.py", line 80, in execute
    result = self.build_and_run()
  File "/home/frappe/benches/bench-2017-03-30/apps/frappe/frappe/model/db_query.py", line 92, in build_and_run
    args = self.prepare_args()
  File "/home/frappe/benches/bench-2017-03-30/apps/frappe/frappe/model/db_query.py", line 110, in prepare_args
    self.build_conditions()
  File "/home/frappe/benches/bench-2017-03-30/apps/frappe/frappe/model/db_query.py", line 240, in build_conditions
    self.build_filter_conditions(self.filters, self.conditions)
  File "/home/frappe/benches/bench-2017-03-30/apps/frappe/frappe/model/db_query.py", line 258, in build_filter_conditions
    conditions.append(self.prepare_filter_condition(f))
  File "/home/frappe/benches/bench-2017-03-30/apps/frappe/frappe/model/db_query.py", line 299, in prepare_filter_condition
    get_datetime(f.value[0]).strftime("%Y-%m-%d %H:%M:%S.%f"),
TypeError: 'NoneType' object has no attribute '__getitem__'

[rmehta]

can you add a test case?