Required "Select options" component with "Other" option always fails if "Other" option is not selected

To reproduce: Create a webform with a component of type "Select options", make it a required field, and set the "Allow "Other..." option" to be checked.

The problem: If the user tries to submit the webform without selecting "Other" and entering a value in the conditionally-displayed text field, the client side validation prevents the form from being submitted.

In Firefox 38.0.5, the small hover-popup "Please fill out this field" appears at the very top left of the browser window.
In Chrome 43.0.2357.132, there is no indication of what the problem is, but the form does not submit.
In Safari 9.07, the client-side validation does not run, so there is no issue.

Workarounds are to make the field not required, or to disable the "Allow Other" feature, neither of which are ideal.