jQuery.data() returns null instead of "" or undefined in 1.3b1

jQuery.data() returns null instead of "" or undefined in 1.3b1


Bug report in tracker:
http://dev.jquery.com/ticket/3748
In jQuery 1.2, when a value is not set or was unset, jQuery.data()
returns undefined.
In jQuery 1.3b1, when a value is not set, it returns null instead of
undefined.
When the "" value (empty string) is stored, it returns null instead of
"".
When null is stored it correctly returns null but it can be confused
with the 2 above cases.