minor #19786 [Testing] Update nullable types (smnandre)

This PR was merged into the 5.4 branch.

Discussion
----------

[Testing] Update nullable types

The Symfony codebase as been recently updated to replace implicit nullable types with explicit ones, but i think the code shown in documentation (method signatures or examples) has not been updated at the same time.

This PR is just a small "example" of what's to be updated on the Testing page

How do you think we should handle this ?
Create a "meta-issue" and split the work by component ?

Commits
-------

c77994d [Testing] Update nullable types

Author: OskarStark

testing.rst

@@ -606,7 +606,7 @@ The full signature of the ``request()`` method is::
         array $parameters = [],
         array $files = [],
         array $server = [],
-        string $content = null,
+        ?string $content = null,
         bool $changeHistory = true
     ): Crawler
 
@@ -1007,18 +1007,18 @@ Response Assertions
     Asserts that the response was successful (HTTP status is 2xx).
 ``assertResponseStatusCodeSame(int $expectedCode, string $message = '')``
     Asserts a specific HTTP status code.
-``assertResponseRedirects(string $expectedLocation = null, int $expectedCode = null, string $message = '')``
+``assertResponseRedirects(?string $expectedLocation = null, ?int $expectedCode = null, string $message = '')``
     Asserts the response is a redirect response (optionally, you can check
     the target location and status code).
 ``assertResponseHasHeader(string $headerName, string $message = '')``/``assertResponseNotHasHeader(string $headerName, string $message = '')``
     Asserts the given header is (not) available on the response, e.g. ``assertResponseHasHeader('content-type');``.
 ``assertResponseHeaderSame(string $headerName, string $expectedValue, string $message = '')``/``assertResponseHeaderNotSame(string $headerName, string $expectedValue, string $message = '')``
     Asserts the given header does (not) contain the expected value on the
     response, e.g. ``assertResponseHeaderSame('content-type', 'application/octet-stream');``.
-``assertResponseHasCookie(string $name, string $path = '/', string $domain = null, string $message = '')``/``assertResponseNotHasCookie(string $name, string $path = '/', string $domain = null, string $message = '')``
+``assertResponseHasCookie(string $name, string $path = '/', ?string $domain = null, string $message = '')``/``assertResponseNotHasCookie(string $name, string $path = '/', ?string $domain = null, string $message = '')``
     Asserts the given cookie is present in the response (optionally
     checking for a specific cookie path or domain).
-``assertResponseCookieValueSame(string $name, string $expectedValue, string $path = '/', string $domain = null, string $message = '')``
+``assertResponseCookieValueSame(string $name, string $expectedValue, string $path = '/', ?string $domain = null, string $message = '')``
     Asserts the given cookie is present and set to the expected value.
 ``assertResponseFormatSame(?string $expectedFormat, string $message = '')``
     Asserts the response format returned by the
@@ -1047,10 +1047,10 @@ Request Assertions
 Browser Assertions
 ..................
 
-``assertBrowserHasCookie(string $name, string $path = '/', string $domain = null, string $message = '')``/``assertBrowserNotHasCookie(string $name, string $path = '/', string $domain = null, string $message = '')``
+``assertBrowserHasCookie(string $name, string $path = '/', ?string $domain = null, string $message = '')``/``assertBrowserNotHasCookie(string $name, string $path = '/', ?string $domain = null, string $message = '')``
     Asserts that the test Client does (not) have the given cookie set
     (meaning, the cookie was set by any response in the test).
-``assertBrowserCookieValueSame(string $name, string $expectedValue, string $path = '/', string $domain = null, string $message = '')``
+``assertBrowserCookieValueSame(string $name, string $expectedValue, string $path = '/', ?string $domain = null, string $message = '')``
     Asserts the given cookie in the test Client is set to the expected
     value.
 ``assertThatForClient(Constraint $constraint, string $message = '')``
@@ -1108,18 +1108,18 @@ Mailer Assertions
     Starting from Symfony 5.1, the following assertions no longer require to make
     a request with the ``Client`` in a test case extending the ``WebTestCase`` class.
 
-``assertEmailCount(int $count, string $transport = null, string $message = '')``
+``assertEmailCount(int $count, ?string $transport = null, string $message = '')``
     Asserts that the expected number of emails was sent.
-``assertQueuedEmailCount(int $count, string $transport = null, string $message = '')``
+``assertQueuedEmailCount(int $count, ?string $transport = null, string $message = '')``
     Asserts that the expected number of emails was queued (e.g. using the
     Messenger component).
 ``assertEmailIsQueued(MessageEvent $event, string $message = '')``/``assertEmailIsNotQueued(MessageEvent $event, string $message = '')``
     Asserts that the given mailer event is (not) queued. Use
-    ``getMailerEvent(int $index = 0, string $transport = null)`` to
+    ``getMailerEvent(int $index = 0, ?string $transport = null)`` to
     retrieve a mailer event by index.
 ``assertEmailAttachmentCount(RawMessage $email, int $count, string $message = '')``
     Asserts that the given email has the expected number of attachments. Use
-    ``getMailerMessage(int $index = 0, string $transport = null)`` to
+    ``getMailerMessage(int $index = 0, ?string $transport = null)`` to
     retrieve a specific email by index.
 ``assertEmailTextBodyContains(RawMessage $email, string $text, string $message = '')``/``assertEmailTextBodyNotContains(RawMessage $email, string $text, string $message = '')``
     Asserts that the text body of the given email does (not) contain the