A blank username should never be allowed.

lib/apiauth.php

@@ -241,7 +241,7 @@ class ApiAuthAction extends ApiAction
             $realm = common_config('site', 'name') . ' API';
         }
 
-        if (!isset($this->auth_user_nickname) && $required) {
+        if (empty($this->auth_user_nickname) && $required) {
             header('WWW-Authenticate: Basic realm="' . $realm . '"');
 
             // show error if the user clicks 'cancel'

lib/util.php

@@ -133,6 +133,11 @@ function common_munge_password($password, $id)
 
 function common_check_user($nickname, $password)
 {
+    // empty nickname always unacceptable
+    if (empty($nickname)) {
+        return false;
+    }
+
     $authenticatedUser = false;
 
     if (Event::handle('StartCheckPassword', array($nickname, $password, &$authenticatedUser))) {