PHP notices with exposed views_handler_filter_string filter in specific cases

Seen with views_handler_filter_string when operator is locked on 'empty' or 'not empty'
(OK, exposing such a filter doesn't make much sense to begin with, since there is then nothing to expose, but the UI still allows it...)

Steps to reproduce (requires CCK, since no in Views built-in filters use the 'allow empty' option) :
- Add a filter on a CCK text field that doesn't have a list of allowed values (because right now those with allowed values use views_handler_filter_many_to_one instead of views_handler_filter_string )
- Expose it, selecting the 'empty' operator, leaving 'unlock operator' unchecked.

notice: Undefined index: value in sites\all\modules\views\handlers\views_handler_filter.inc on line 416.
notice: Undefined index: field_example_1_value in sites\all\modules\views\handlers\views_handler_filter.inc on line 501.
(field_example_1 is the name of the CCK field)

Adding $form['value'] = array(); at the beginning of views_handler_filter_string::value_form() solves the first error, but not the second.