Change behaviour if authorizationToUser is null

authorizationToUser() returns now an empty string by default
If it returns an empty string, conf is set to null and api don't die
anymore.

Fix https://github.com/FreshRSS/FreshRSS/issues/747

p/api/greader.php

@@ -168,7 +168,7 @@ function authorizationToUser() {
 			}
 		}
 	}
-	return null;
+	return '';
 }
 
 function clientLogin($email, $pass) {	//http://web.archive.org/web/20130604091042/http://undoc.in/clientLogin.html
@@ -552,10 +552,10 @@ if (!$system_conf->api_enabled) {
 Minz_Session::init('FreshRSS');
 
 $user = authorizationToUser();
-if (is_null($user)) {
-	unauthorized();
+$conf = null;
+if ($user !== '') {
+	$conf = get_user_configuration($user);
 }
-$conf = get_user_configuration($user);
 
 logMe('User => ' . $user . "\n");