Re-enable document compatibility with 2.4

But only if there aren't any compositions. Krita 2.4 can only load
documents where there is nothing else but layers in the image; 2.5
adds compositions and master adds assistants.

So, if there are no compositions, don't save an empty composition tag,
then 2.4 can still load the document. If there are compositions or
assistants, it's tant-pis...