fix php warning

Author: gimler

cookbook/doctrine/file_uploads.rst

@@ -188,7 +188,7 @@ object, which is what's returned after a ``file`` field is submitted::
  public function upload()
  {
  // the file property can be empty if the field is not required
- if (!$this->file) {
+ if (empty($this->file)) {
  return;
  }
 
@@ -245,7 +245,7 @@ Next, refactor the ``Document`` class to take advantage of these callbacks::
  */
  public function preUpload()
  {
- if ($this->file) {
+ if (!empty($this->file)) {
  // do whatever you want to generate a unique name
  $this->setPath(uniq().'.'.$this->file->guessExtension());
  }
@@ -256,7 +256,7 @@ Next, refactor the ``Document`` class to take advantage of these callbacks::
  */
  public function upload()
  {
- if (!$this->file) {
+ if (empty($this->file)) {
  return;
  }
 
@@ -303,7 +303,7 @@ property, instead of the actual filename::
  */
  public function preUpload()
  {
- if ($this->file) {
+ if (!empty($this->file)) {
  $this->setPath($this->file->guessExtension());
  }
  }
@@ -313,7 +313,7 @@ property, instead of the actual filename::
  */
  public function upload()
  {
- if (!$this->file) {
+ if (empty($this->file)) {
  return;
  }