Link with `null` attribute

[Matthew Weier O'Phinney]

Creating a Link with a null attribute, I get an InvalidArgumentException. I'm not 100% sure about it but I guess that having null attributes should be allowed in Hal links

Code to reproduce the issue

new Link('a', 'b', false, ['c' => null]);

Expected results

I would expect to get a Link with a c attribute with a null value

Actual results

An InvalidArgumentException with message Zend\\Expressive\\Hal\\Link expects the $value to be a PHP primitive or array of strings; received NULL is thrown

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Originally posted by @marcosh at zendframework/zend-expressive-hal#46

[Aleksei Khudiakov]

Latest draft for HAL defines Link object with attributes that are optional but not empty.
https://datatracker.ietf.org/doc/html/draft-kelly-json-hal-08#section-5

I do not think attributes with null values should be allowed.