Dispatch change event for input type=checkbox when checked but value is null

Checkbox dispatches change event when checked and dispatch event is not
based on value being null or not.

BUG=366989
R=tkent,keishi1
Test=If value attribute is null, when checkbox is checked a change event
is triggered.

Review URL: https://codereview.chromium.org/255163002

git-svn-id: svn://svn.chromium.org/blink/trunk@173530 bbb929c8-8fbe-4397-9dbb-9b2b20218538