[6.x] Add assertNoContent to TestResponse

[jasonmccreary]

This assertion is similar to the recently added assertForbidden and assertUnauthorized methods. However, its compound assertion provides an easier way to test for "no content" responses while also specifying an optional status code.

Before

$response = $this->get('api/endpoint');

$response->assertStatus(409);
$this->assertEmpty($response->content());

After

$response = $this->get('api/endpoint');

$response->assertNoContent(409);

[dwightwatson]

This feels a little odd to me in the sense that all the other assert{$httpStatus} helper methods literally only assert the status - making this one an outlier in that it can take an argument, and that it has an additional expectation.

Would it be better to also introduce assertEmpty so could be used separately (and also be a bit more explicit)?

$response->assertStatus(409)->assertEmpty();

$response->assertNoContent()->assertEmpty();

[jasonmccreary]

@dwightwatson this is the exact same behavior of response()->noContent() which would be used in the implementation. It too takes an optional argument of the status code.

It's important to remember a "no content" response by default is not just a 204 status code, but also an a response with "no body". As such, the compound expression is what makes this assertion improve the developer experience.

With that said, a $response->assertEmpty() might be helpful depending on if this is merged or not.

Changes made.