fix(symfony): upgrade command requires diff

fixes #4950

Author: soyuka

src/Core/Bridge/Symfony/Bundle/Command/UpgradeApiResourceCommand.php

@@ -142,6 +142,12 @@ protected function execute(InputInterface $input, OutputInterface $output): int
                     continue;
                 }
 
+                if (!class_exists(Differ::class)) {
+                    $output->writeln('Run `composer required --dev sebastian/diff` or install phpunit to print a diff.');
+
+                    return Command::FAILURE;
+                }
+
                 $this->printDiff($oldCode, $newCode, $output);
                 continue;
             }