^3.0 compatibility with dependency broken

[soundsgoodsofar]

In zendframework/zend-diactoros 1.8.6, they made a breaking change in src/MessageTrait.php

zendframework/zend-diactoros@ee4bcdc#diff-5abca4d9d5693da46d10a54795b1192d

This change means that the following line in ServerBag.php results in an exception being thrown for an empty header.

$headers['PHP_AUTH_PW'] = isset($this->parameters['PHP_AUTH_PW']) ? $this->parameters['PHP_AUTH_PW'] : '';

Propose to change that line to:

if (isset($this->parameters['PHP_AUTH_PW'])) {
    $headers['PHP_AUTH_PW'] = $this->parameters['PHP_AUTH_PW'];
}

[driesvints]

This line seems to be in place in Symfony's HttpFoundation's ServerBag class. So it would have to be fixed there instead.