Generate state only if empty

[jaybiedev71]

This is similar to this issue on nonce (#452),

I extended the class to be able to set custom state (and nonce) to be passed to adb2c, but this gets overwritten in requestAuthorization() method. We can update code to generate only state when it is empty. Be glad to issue a PR if you prefer.

OpenID-Connect-PHP/src/OpenIDConnectClient.php

Line 750 in f7c91b9

$state = $this->setState($this->generateRandString());

    $state = $this->getState();
    if (empty($state)) {
        $state = $this->setState($this->generateRandString());
   }