Change xhr to survive uriPolicy returning null instead of undefined

The uri policy may return falsy values other than undefined (for example null).  The test in
xhr should only use the value return by the url policy if it is a string.

I considered testing for falsy but "" may be a valid uri for the policy to return.

@4367

[Mark S. Miller, 2011-02-04 18:56:25]

http://codereview.appspot.com/4129051/diff/1/src/com/google/caja/demos/playgr...
File src/com/google/caja/demos/playground/taming.js (right):

http://codereview.appspot.com/4129051/diff/1/src/com/google/caja/demos/playgr...
src/com/google/caja/demos/playground/taming.js:38: if
(!/^https?:\/\//i.test(uri)) { return undefined; }
void 0 ?

[Jasvir, 2011-02-04 18:58:21]

http://codereview.appspot.com/4129051/diff/1/src/com/google/caja/demos/playgr...
File src/com/google/caja/demos/playground/taming.js (right):

http://codereview.appspot.com/4129051/diff/1/src/com/google/caja/demos/playgr...
src/com/google/caja/demos/playground/taming.js:38: if
(!/^https?:\/\//i.test(uri)) { return undefined; }
On 2011/02/04 18:56:25, Mark S. Miller wrote:
> void 0 ?

Done.

[Jasvir, 2011-02-08 11:50:01]

ping

[Mark S. Miller, 2011-02-08 15:25:13]

LGTM