Fix pixlr not able to handle nullable extension

[jordisala1991]

Subject

I am targeting this branch, because this is a bugfix.

Backporting directly from master branch.

Closes #1670 .

Changelog

### Fixed
- Fixed adding new Media with Pixlr enabled.

[franmomu]

Shouldn't this be updated as well?

SonataMediaBundle/src/Model/MediaInterface.php

Lines 324 to 327 in 8af42a1

	/**
	* @return string
	*/
	public function getExtension();

[jordisala1991]

Shouldn't this be updated as well?

SonataMediaBundle/src/Model/MediaInterface.php

Lines 324 to 327 in 8af42a1

	/**
	* @return string
	*/
	public function getExtension();

@phansys is already modifying the phpdoc at other Pr.