Crash with eZ Media FieldType

I don't know exactly why or where, but the legacy kernel ( in admin interface ) add an empty line in ezmedia table when an object is created.

But this empty line isn't correctly handle by getFieldData, $field->value->externalData['id'] is equal to "/".

So we enter in :

``` php
        // @todo An absolute path can in no case be loaded, but throwing an exception is a bit too much at this stage
        if ( $binaryFileId[0] === '/' )
            return false;
```
In IOService.php line 182.

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