fix(tests): make required params strings

Author: jankapunkt

lib/grant-types/authorization-code-grant-type.js

@@ -163,7 +163,7 @@ class AuthorizationCodeGrantType extends AbstractGrantType {
 
     const redirectUri = request.body.redirect_uri || request.query.redirect_uri;
 
-    if (!isFormat.uri(redirectUri)) {
+    if (!redirectUri || !isFormat.uri(redirectUri)) {
       throw new InvalidRequestError('Invalid request: `redirect_uri` is not a valid URI');
     }