No need to check for uniqueness of username if none has been provided

src/Auth.php

@@ -719,6 +719,8 @@ class Auth {
 
 		// if the uniqueness of the username is to be ensured
 		if ($requireUniqueUsername) {
+			// if a username has actually been provided
+			if ($username !== null) {
 				// count the number of users who do already have that specified username
 				$occurrencesOfUsername = $this->db->selectValue(
 					'SELECT COUNT(*) FROM users WHERE username = ?',
@@ -731,6 +733,7 @@ class Auth {
 					throw new DuplicateUsernameException();
 				}
 			}
+		}
 
 		$password = password_hash($password, PASSWORD_DEFAULT);
 		$verified = isset($callback) && is_callable($callback) ? 0 : 1;