[PHP] [ci skip] Add nullable logic

Created by: githubERIK

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\. If contributing template-only or documentation-only changes which will change sample output, be sure to build the project first.
• Filed the PR against the correct branch: master, 4.1.x, 5.0.x. Default: master. Master should be 🆗.
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Description of the PR

Fix https://github.com/OpenAPITools/openapi-generator/issues/2199

How to check the outcome

• Create an account → https://dashboard.messente.com/register
• Setup API username and password → https://messente.com/documentation/api-setup (click the green "Update" button to start using the credentials)
• Create a new composer project and run composer require messente/messente-api-php
• Use the following code with correct credentials

<?php
require_once __DIR__ . '/vendor/autoload.php';
use Messente\Api\Configuration;
use GuzzleHttp\Client;
use Messente\Api\Api\ContactsApi;
use Messente\Api\Model\ContactUpdateFields;

$config = Configuration::getDefaultConfiguration();
$config->setUsername('____');
$config->setPassword('____');

$api = new ContactsApi(new Client(), $config);

try {
  $response = $api->createContact([
    'phoneNumber' => '+37251000000',
    'email' => 'anyone@messente.com',
    'firstName' => 'Any',
    'lastName' => 'One',
    'company' => 'Messente',
    'title' => 'Sir',
    'custom' => 'Any custom',
    'custom2' => 'Any custom two',
    'custom3' => 'Any custom three',
    'custom4' => 'Any custom four'
  ]);
  echo $response.PHP_EOL;
  echo '----'.PHP_EOL;

  $response = $api->fetchContact('+37251000000');
  echo $response.PHP_EOL;
  echo '----'.PHP_EOL;

  $updateFields = new ContactUpdateFields([
    'lastName' => 'Johnson',
    'custom2' => null,
  ]);
  $response = $api->updateContact('+37251000000', $updateFields);
  echo $response.PHP_EOL;
  echo '----'.PHP_EOL;

} catch (Exception $e) {
  echo 'Exception when calling the API: ', $e->getMessage(), PHP_EOL;
}
?>

and given "email", "firstName","lastName", "company", "title", "custom", "custom2","custom3", "custom4" are nullable:true check that ...

When a request goes out

• field is nullable and value is null
  • ✅ when somebody set the value to null on purpose then this should not be filtered out by ObjectSerializer
  • ✅ when somebody did not specify the value when using the constructor (causing the constructor to automatically set the value to null) then that null value should be filtered out

When a response comes in

• field is nullable and value is null
  • ✅ echo $response should show all null values.