PHP 8.1: fix deprecation warnings / http_build_query()

This fixes all relevant calls to the PHP native `http_build_query()` function.
The second parameter of which is the _optional_ `$numeric_prefix` parameter which expects a `string`.

A parameter being optional, however, does not automatically make it nullable.

As of PHP 8.1, passing `null` to a non-nullable PHP native function will generate a deprecation notice.
In this case, these function calls yielded a `http_build_query(): Passing null to parameter #2 ($numeric_prefix) of type string is deprecated` notice.

Changing the `null` to an empty string fixes this without BC-break.

Fixes a few deprecation warnings found when running the tests.

Refs:
* https://www.php.net/manual/en/function.http-build-query.php
* https://wiki.php.net/rfc/deprecate_null_to_scalar_internal_arg

src/Composer/Repository/Vcs/BitbucketDriver.php

@@ -86,7 +86,7 @@ protected function getRepoData()
             $this->repository,
             http_build_query(
                 array('fields' => '-project,-owner'),
-                null,
+                '',
                 '&'
             )
         );
@@ -286,7 +286,7 @@ public function getTags()
                         'fields' => 'values.name,values.target.hash,next',
                         'sort' => '-target.date',
                     ),
-                    null,
+                    '',
                     '&'
                 )
             );
@@ -330,7 +330,7 @@ public function getBranches()
                         'fields' => 'values.name,values.target.hash,values.heads,next',
                         'sort' => '-target.date',
                     ),
-                    null,
+                    '',
                     '&'
                 )
             );

src/Composer/Util/GitLab.php

@@ -172,7 +172,7 @@ private function createToken($scheme, $originUrl)
             'username' => $username,
             'password' => $password,
             'grant_type' => 'password',
-        ), null, '&');
+        ), '', '&');
         $options = array(
             'retry-auth-failure' => false,
             'http' => array(