assertJsonStringEqualsJsonString empty object matches empty array

[SjorsO]

Q	A
PHPUnit version	6.4.4
PHP version	7.1.10
Installation Method	Composer

I ran into this issue a few days ago, the following tests pass:

$this->assertJsonStringEqualsJsonString('{}', '[]');

// The test still passes if the object/array is nested deeper
$this->assertJsonStringEqualsJsonString('{"key":{}}', '{"key":[]}');

[mkasberg]

I was able to reproduce the problem, and I opened a pull request with a possible solution.