A bug was discovered on the New Review Request page where using the

repositories search bar to filter the repositories and then selecting one would

break uploading diff files. Instead of being redirected to the review request

draft page after uploading a diff, the page would not redirect and the uploader

would just display a loading spinner. This happens because using the search bar

causes the repositories' localSitePrefix attribute to be set to null

instead of an empty string. Then when sending a request to validate the diff,

our ValidateDiffModel would build the request URL by prepending the

localSitePrefix to a api/validation/diffs/ URI. So when the

localSitePrefix is null this would result in a request being made to the

nullapi/validation/diffs/ URL, which does not exist.
This change fixes this by making sure the localSitePrefix is replaced with

an empty string if it is null when building the URL. This is consistent with

the way we handle the localSitePrefix for URL building in our other

Javascript models.

Manually tested uploading diff files with and without using the search bar

  to filter the repositories.
Added a JS unit test for this and ran all JS unit tests.