Do not expand undefined URI template variables.

[gabesullice]

Hello 👋! I am really excited to have found that the league has an implementation of [RFC 6570 - URI Template](https://tools.ietf.org/html/rfc6570#section-3.2.1, especially since the Guzzle implementation will be removed. However, when I changed from Guzzle's implementation to this one, I think I found a slight bug/incompatibility.

RFC6570 states that:

A variable that is undefined (Section 2.3) has no value and is ignored by the expansion process. If all of the variables in an expression are undefined, then the expression's expansion is the empty string. (emphasis mine)

Therefore, if I wrote the following:

$template = new UriTemplate('{?x,y,undef}');
$params = [
  'x' => 'foo',
  'y' => 42,
];
echo (string) $template->expand($params), PHP_EOL;

I would expect the expanded URI to be:

?x=foo&y=42

... given that the variable undef is not defined in my parameter array.

However, what I actually see is that the URI is expanded to:

?x=foo&y=42&undef=

For comparison, this online URI template tester gives my expected results: https://spooky.github.io/uri-template-tester/

You can try it with:
http://example.org/test{?x,y,undef}

{
  "x": "foo",
  "y": 42
}

You should get http://example.org/test?x=foo&y=42 as shown below.

[nyamsprod]

@gabesullice I'll tentatively accept the PR but I will more than probably change some part of the code but the bugfix will be reported as fixed by your contribution thanks for the report and the PR.

[gabesullice]

Wow, thanks for the quick turnaround @nyamsprod! I assumed the code would change :) it was just the simplest, fastest way to add a test and a fix along with the bug report. Thanks again!