Show delete field only if there is a file

Just add some conditions to VichFileType to show delete field only if there is a file.
Not in FileSystemStorage.php

```
    public function resolveUri($obj, $mappingName, $className = null)
    {
        list($mapping, $name) = $this->getFilename($obj, $mappingName, $className);

        $uriPrefix = $mapping->getUriPrefix();
        $uploadDir = $mapping->getUploadDestination() . $mapping->getUploadDir($obj);
        $parts = explode($uriPrefix, $this->convertWindowsDirectorySeparator($uploadDir));

        return sprintf('%s/%s', $uriPrefix . array_pop($parts), $name);
    }
```