[5.7] allow 'assertJsonMissingValidationErrors' to be called without an argument

[SjorsO]

This PR is a different implementation of #27172

This PR makes calling assertJsonMissingValidationErrors without an argument behave the same as as assertSessionHasNoErrors() (but it works for Json responses, since they don't store their errors in session).

Example:

$this->post(route('api.users.store'), $data)
    ->assertJsonMissingValidationErrors()
    ->assertStatus(201);

If this test has validation errors, it will print them to the console. If you would write the same test without the assertJsonMissingValidationErrors(), and then you get validation errors, the test will only print an unhelpful "expected status 201 but received 422".