[Debug] Set exit status to 255 on error

This PR was submitted for the 2.8 branch but it was merged into the 2.7 branch instead (closes #22307).

Discussion
----------

[Debug] Fix php notice

| Q             | A
| ------------- | ---
| Branch?       | 2.8
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        |

Of course autoloading of an empty string should not actually happen (fixed that in https://github.com/twigphp/Twig/pull/2438) but if it does happen it should not throw a php notice.

```
Notice: Uninitialized string offset 0
```

Commits
-------

e333a1a [Debug] Fix php notice

DebugClassLoader.php

@@ -186,7 +186,7 @@ class DebugClassLoader
 
         $exists = class_exists($class, false) || interface_exists($class, false) || (function_exists('trait_exists') && trait_exists($class, false));
 
-        if ('\\' === $class[0]) {
+        if ($class && '\\' === $class[0]) {
             $class = substr($class, 1);
         }
 

ErrorHandler.php

@@ -101,6 +101,7 @@ class ErrorHandler
     private static $reservedMemory;
     private static $stackedErrors = array();
     private static $stackedErrorLevels = array();
+    private static $exitCode = 0;
 
     /**
      * Same init value as thrownErrors.
@@ -477,6 +478,9 @@ class ErrorHandler
      */
     public function handleException($exception, array $error = null)
     {
+        if (null === $error) {
+            self::$exitCode = 255;
+        }
         if (!$exception instanceof \Exception) {
             $exception = new FatalThrowableError($exception);
         }
@@ -562,7 +566,7 @@ class ErrorHandler
             return;
         }
 
-        if (null === $error) {
+        if ($exit = null === $error) {
             $error = error_get_last();
         }
 
@@ -586,15 +590,21 @@ class ErrorHandler
             } else {
                 $exception = new FatalErrorException($handler->levels[$error['type']].': '.$error['message'], 0, $error['type'], $error['file'], $error['line'], 2, true, $trace);
             }
-        } elseif (!isset($exception)) {
-            return;
         }
 
         try {
-            $handler->handleException($exception, $error);
+            if (isset($exception)) {
+                self::$exitCode = 255;
+                $handler->handleException($exception, $error);
+            }
         } catch (FatalErrorException $e) {
             // Ignore this re-throw
         }
+
+        if ($exit && self::$exitCode) {
+            $exitCode = self::$exitCode;
+            register_shutdown_function('register_shutdown_function', function () use ($exitCode) { exit($exitCode); });
+        }
     }
 
     /**

phpunit.xml.dist

@@ -5,6 +5,8 @@
          backupGlobals="false"
          colors="true"
          bootstrap="vendor/autoload.php"
+         failOnRisky="true"
+         failOnWarning="true"
 >
     <php>
         <ini name="error_reporting" value="-1" />