authPrompt: don't allow next if entry is empty

Normally the user isn't allowed to proceed passed
the username question until they've filled it in.
To ensure this, the authprompt code desensitizes
the next button when the number of characters change to
zero.

Unfortunately it fails to desensitize the next button
up front when the entry starts out empty.

This commit addresses that bug.

https://bugzilla.gnome.org/show_bug.cgi?id=752739

js/gdm/authPrompt.js

@@ -401,7 +401,7 @@ const AuthPrompt = new Lang.Class({
     },
 
     updateSensitivity: function(sensitive) {
-        this._updateNextButtonSensitivity(sensitive);
+        this._updateNextButtonSensitivity(sensitive && this._entry.text.length > 0);
         this._entry.reactive = sensitive;
         this._entry.clutter_text.editable = sensitive;
     },