Initialize variable to prevent error message

If ExtendedSearch is enabled, but the service is not available,
$http_response_header will not be set by file_get_contents, as expected.
This leads to a notice that breaks the json, e.g. for the statebar
content. Initializing the variable helps here.

Change-Id: Ib39bbc7b8b1970fd0a708296c1b671182e9929a2