Indicate possible null values in PHPDoc @return (#1097)
Indicate possible null values in PHPDoc @return (#1097)

To prevent error messages from PHPStan like:

```
Method                                                                  
         MyProject\Domain\Bot::getChatUsername()   
         never returns null so it can be removed from the return typehint.
```

when using code like this:
```php
class Bot{

    /** @var BotManApiClient */
    private $botMan;

    public function getChatUsername() : ?string
    {
        return $this->botMan->getUser()->getUserName();
    }
}
```
botman/botman