OEmbed ProviderRepositoryTest::testEmptyProviderList() does not interact with Guzzle's API correctly

Problem/Motivation

Drupal\Tests\media\Functional\ProviderRepositoryTest::testEmptyProviderList() has the following code:

    $response = $this->prophesize('\GuzzleHttp\Psr7\Response');
    $response->getBody()->willReturn($content);

    $client = $this->createMock('\GuzzleHttp\Client');
    $client->method('request')->withAnyParameters()->willReturn($response->reveal());
    $this->container->set('http_client', $client);

    $this->expectException(ProviderException::class);
    $this->expectExceptionMessage('Remote oEmbed providers database returned invalid or empty list.');
    $this->container->get('media.oembed.provider_repository')->getAll();

The data provider for the method tests two values of $content, an empty array and an empty string.

This is problematic, because \GuzzleHttp\Psr7\Response::getBody() is documented to only return either StreamInterface or NULL. It cannot return an empty string or an empty array, so the test is not actually testing the intended case of an empty JSON response. It's instead testing impossible return values for the response body getter.

This issue is major because it blocks #3220220: Update guzzlehttp/psr7 to 2.1.0, as the 2.x branch of the Guzzle psr7 package adds an actual return typehint, so the test will start failing explicitly with a TypeError instead of the intended exception.

Proposed resolution

Change the test to actually test a response stream containing empty JSON values.

Remaining tasks

Rebase #3220220: Update guzzlehttp/psr7 to 2.1.0 on top of this once this lands.