Fix #35269: compositing setup with only file output node and no composite node
refused to render. It's not possible to actually see the compositing result
in the render result without that but might as well work.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56655 954f8c5b-7b00-dc11-b283-0030488c597c