Set related record id to null if related record is empty

Author: phpfui

Diff for: Tests/Unit/RecordTest.php

@@ -30,6 +30,6 @@ public function testEmpty() : void
 		$this->assertFalse(empty($order->employee_id));
 		$order->employee = new \Tests\Fixtures\Record\Employee();
 		$this->assertTrue(empty($order->employee));
-		$this->assertTrue(empty($order->employee_id));
+		$this->assertNull($order->employee_id);
 		}
 	}

Diff for: src/PHPFUI/ORM/Record.php

@@ -157,7 +157,7 @@ public function __set(string $field, mixed $value) : void
 				{
 				if ($value->empty())
 					{
-					$this->current[$id] = 0;
+					$this->current[$id] = static::$fields[$id]->nullable ? null : 0;
 
 					return;
 					}