/api/issues/bulk_change only accepts a comment parameter if some other action is performed as well (change assignee, change type, etc). If this isn't provided, the API returns an error.
In the UI however, nothing prevents a user from submitting the form by only setting a comment. This results in an error:
The UI should always be aligned on the underlying WS.
Disable the form submission as long as no "change" other than a comment has been configured.