minor #40271 Switched to non-null defaults in exception constructors (derrabus)

This PR was merged into the 4.4 branch.

Discussion
----------

Switched to non-null defaults in exception constructors

| Q             | A
| ------------- | ---
| Branch?       | 4.4
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| Tickets       | N/A
| License       | MIT
| Doc PR        | N/A

PHP 8.1 will trigger a deprecation warning if we pass `null` as `$message` or `$code` to the constructor of `\Exception`. However, many of our own exception accept `null` for those parameters and even use them as default.

This is unfortunate because code like the following snippet would trigger that deprecation although the code itself is perfectly fine:

```php
throw new NotFoundHttpException();
```

With this PR, I'd like to change our defaults to `''` and `0` while still allowing to pass `null` for BC. In a follow-up PR for the 5.x branch, I'd like to deprecate passing `null`, matching the future behavior of PHP.

This PR also adjust various PHPDoc blocks with inaccurate types.

Commits
-------

f8e10094a4 Switched to non-null defaults in exception constructors

Author: nicolas-grekas

Exception/MethodNotAllowedException.php

@@ -22,7 +22,10 @@ class MethodNotAllowedException extends \RuntimeException implements ExceptionIn
 {
     protected $allowedMethods = [];
 
-    public function __construct(array $allowedMethods, string $message = null, int $code = 0, \Throwable $previous = null)
+    /**
+     * @param string[] $allowedMethods
+     */
+    public function __construct(array $allowedMethods, ?string $message = '', int $code = 0, \Throwable $previous = null)
     {
         $this->allowedMethods = array_map('strtoupper', $allowedMethods);
 
@@ -32,7 +35,7 @@ public function __construct(array $allowedMethods, string $message = null, int $
     /**
      * Gets the allowed HTTP methods.
      *
-     * @return array
+     * @return string[]
      */
     public function getAllowedMethods()
     {

RouteCollectionBuilder.php

@@ -362,11 +362,11 @@ private function load($resource, string $type = null): array
         }
 
         if (null === $resolver = $this->loader->getResolver()) {
-            throw new LoaderLoadException($resource, null, null, null, $type);
+            throw new LoaderLoadException($resource, null, 0, null, $type);
         }
 
         if (false === $loader = $resolver->resolve($resource, $type)) {
-            throw new LoaderLoadException($resource, null, null, null, $type);
+            throw new LoaderLoadException($resource, null, 0, null, $type);
         }
 
         $collections = $loader->load($resource, $type);