Adds ?integer, ?double, ?string, etc. cast types :)

Adds new cast types which are casting only if value !== null

i.e. I've got an nullable col in my dbtable which stores strings (empty or not) and I'd like to have possibility to determine if string is null or '';
Now I cannot when I define casting as a string. with this PR if I define ?string in 

```

	protected $_options = [
		'casts' => [
			'usr_id' 			=> 'integer',
			'usr_usp_id' 		=> 'integer',
			'usr_provider_id' 	=> 'string',

			'usr_exc_id'		=> 'integer',
			'usr_drs_id'		=> 'integer',
			'usr_dlc_id'		=> '?string',
			'usr_tzo_id'		=> 'integer',

			'usr_email' 		=> 'string',
			'usr_password' 		=> 'string',

			'usr_first_name' 	=> 'string',
			'usr_last_name' 	=> 'string',
			'usr_nick_name'		=> 'string',

			'usr_is_active' 	=> 'boolean',
			'usr_is_blocked' 	=> 'boolean',
			'usr_is_picture' 	=> 'boolean',
		],
		'dates'=> ['usr_created_at', 'usr_updated_at'],
		'datamap' => []
	];
```
I will get NULL or '' for usr_dlc_id according to database value.

Author: nowackipawel

system/Entity.php

@@ -421,6 +421,8 @@ protected function mutateDate($value)
 
 	protected function castAs($value, string $type)
 	{
+		if(substr($type,0,1) === '?' && $value === null) return null;
+
 		switch($type)
 		{
 			case 'integer':