js to hide payment block on events doesn't account for text input fields

when you setup a paid event, the payment details blocks are hidden by default. if a fee option is selected that has a value (i.e. != 0), the block is displayed.

this works well for radio button fields and checkbox fields, but not for text input fields. for example:
http://drupal.demo.civicrm.org/civicrm/event/register?reset=1&id=7

if you choose the $0 option for the radio button field, and then enter a value into the text field, the billing block remains hidden – preventing you from submitting the form (since the required fields in that block are not available to be completed)