bug #37327 [HttpFoundation] Allow null in InputBag@set (taylorotwell)

This PR was squashed before being merged into the 5.1 branch (closes #37327).

Discussion
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[HttpFoundation] Allow `null` in InputBag@set

This allows `null` to be passed to InputBag's `set` method.

| Q             | A
| ------------- | ---
| Branch?       | 5.1
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| License       | MIT

Previously, `null` was an allowed value to the InputBag's `set` method. At some point this was deprecated. It would be very nice for us to be able to set this to `null`. For example, this ability drives a very popular feature of Laravel where we ship a middleware that converts all empty strings to `null` on incoming requests.

Note that the `get` method already specifically allows `null` and `null` is a valid decoded JSON value.

Commits
-------

14ec6a7 [HttpFoundation] Allow `null` in InputBag@set

src/Symfony/Component/HttpFoundation/InputBag.php

@@ -72,12 +72,12 @@ public function add(array $inputs = [])
     /**
      * Sets an input by name.
      *
-     * @param string|array $value
+     * @param string|array|null $value
      */
     public function set(string $key, $value)
     {
-        if (!is_scalar($value) && !\is_array($value) && !method_exists($value, '__toString')) {
-            trigger_deprecation('symfony/http-foundation', '5.1', 'Passing "%s" as a 2nd Argument to "%s()" is deprecated, pass a string or an array instead.', get_debug_type($value), __METHOD__);
+        if (null !== $value && !is_scalar($value) && !\is_array($value) && !method_exists($value, '__toString')) {
+            trigger_deprecation('symfony/http-foundation', '5.1', 'Passing "%s" as a 2nd Argument to "%s()" is deprecated, pass a string, array, or null instead.', get_debug_type($value), __METHOD__);
         }
 
         $this->parameters[$key] = $value;

src/Symfony/Component/HttpFoundation/Tests/InputBagTest.php

@@ -51,7 +51,7 @@ public function testFilterArray()
     public function testSetWithNonStringishOrArrayIsDeprecated()
     {
         $bag = new InputBag();
-        $this->expectDeprecation('Since symfony/http-foundation 5.1: Passing "Symfony\Component\HttpFoundation\InputBag" as a 2nd Argument to "Symfony\Component\HttpFoundation\InputBag::set()" is deprecated, pass a string or an array instead.');
+        $this->expectDeprecation('Since symfony/http-foundation 5.1: Passing "Symfony\Component\HttpFoundation\InputBag" as a 2nd Argument to "Symfony\Component\HttpFoundation\InputBag::set()" is deprecated, pass a string, array, or null instead.');
         $bag->set('foo', new InputBag());
     }