Hi,
Thanks for pointing that out. Its actually an image button that is intended to be hidden and not used directly by a user. The purpose of the button is to to add a global content instance to the page when the add global content dialog is dismissed. If you configure an html instance to be global so it can be re-used on multiple pagres then a new link appears on this page (PageLayout.aspx) that links to the global content dialog page. The link is not shown uless there exists global content that hasn't been added to the current page. Then if you choose content from the global dialog upon dismissing the dialog javascript will set a hidden field with the module id of the selected content then it will click this hidden button to add it to the page. So its a button intedned to be clicked only from javascript not by the user.
I will fix this for the next release by adding tabIndex="-1" on the hidden button, this well prevent you from tabbing into the hidden image button so it will avoid the confusion about the blank label.
Thanks,
Joe