Allow UploadedFile to be of size zero

[Zegnat]

Q	A
Bug fix?	yes
New feature?	no
BC breaks?	no?
Deprecations?	no
Related tickets	none
Documentation	n/a
License	MIT

What's in this PR?

Patch to allow an UploadedFile instance to pass testing when it returns a (valid) integer of 0 (zero) as its size.

Why?

I ran into a problem when I was giving the integration tests an UploadedFile instance with an empty Stream within. At this point getSize() will return 0, which does not pass the first if, and therefor fails the test as 0 !== null.

Example Usage

class UploadedFileTest extends \Http\Psr7Test\UploadedFileIntegrationTest
{
    public function createSubject()
    {
        $stream = (new \Nyholm\Psr7\Factory\Psr17Factory())->createStream();
        return (new \Nyholm\Psr7\Factory\Psr17Factory())->createUploadedFile($stream);
    }
}

// 1) UploadedFileTest::testGetSize
// Failed asserting that 0 is null.

[dbu]

thank you!