Make sure that image reference is false if ini setting 'Reference' is empty

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:
```ini
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.
```

Cheers
:octocat: Jérôme