Update JoinColumn.php

If $referencedColumnName = 'id' by default, it doesn't make sense to make checks like:
```php
if (empty($joinColumn['referencedColumnName'])) {
```
(ClassMetaDataInfo file)

-----
Considering you map your column
```
@ORM\JoinColumn(onDelete="CASCADE")
```
You'll get JoinColumn with 'id' value, which doesn't let doctrine use naming strategies for referenced column names