Fixes SQLServer throwing a 'string not uuid' type error if a query is done with a empty value to a uuid field.

Example:

I have a table with a `uniqueidentifier` primary key:
```php
$table->find()->where(['id' => null]);
```
Will work correctly, however
```php
$table->find()->where(['id' => '']);
```
Will throw a PDOException with `Conversion failed when converting from a character string`.

This causes problems with, for example, association form fields with a 'empty' default value.

I've added an extra check in the UuidType class to map empty strings to `null` values.