Fix EZP-23477: exception when trashing empty image

> Fixes [EZP-23477](https://jira.ez.no/browse/EZP-23477)

```
Argument 'BinaryFile::id' is invalid: '' is wrong value in class 'BinaryFile'
```
Fixed by only calling `ezpEvent::getInstance()->notify( 'image/trashAliases', array( $originalAlias['url'] ) );` if there is an actual image to trash aliases for.
is cast done before comparison? for clarity we should add extra parentheses here.