JsonItemStore url parameter needs to be declared as a string

[bill]

For the parser's benefit, to know that url is a String, file needs to say

url: ""

rather than

url: null

This is so we can use JsonItemStore? declaratively, like

<div dojoType=dojo.data.JsonItemStore url="foo.json">

which causes the parser to do

eval("var tmp=foo.json")

which is missing quotes around the file name, and causes problems (at least on my machine) in test_Combobox.html.

[bill]

(In [9320]) Declare url as a string for the benefit of the parser. Fixes #3539