This PR checks if the setting Reference= in image.ini is empty and sets the corresponding alias setting in eZImageManager to false instead of an empty string (which is the current behaviour).

The default image aliases in image.ini have the reference image alias as reference. Many (as well as eZFlow and eZMultiUpload) override this by setting the Reference variable to an empty value:

Reference=

This has the effect that the image alias settings will receive an empty string instead of the expected false which will create an unneeded entry in the error.log:

The referenced alias '' for image alias 'my_alias' does not exist, cannot use it for reference.
Will use 'original' alias instead.