Pass '' instead of null to http_build_query()'s second parameter

[TimWolla]

Passing null will show a deprecation warning with PHP 8.1.

Q	A
Bug fix?	yes
New feature?	no
BC breaks?	no
Deprecations?	no
Related tickets	n/a
Documentation	n/a
License	MIT

What's in this PR?

This fixes a deprecation warning in PHP 8.1

Why?

Deprecated: http_build_query(): Passing null to parameter #2 ($numeric_prefix) of type string is deprecated in /pwd/vendor/php-http/client-integration-tests/src/HttpBaseTest.php on line 195

Example Usage

n/a

Checklist

• Updated CHANGELOG.md to describe BC breaks / deprecations | new feature | bugfix
• Documentation pull request created (if not simply a bugfix)

To Do

n/a

[dbu]

the CI seems to have a problem, but its not related to this.

the change seems sane to me, thanks