Image warmer doesn't handle empty image fields

[Bartvds]

The image warmer assumes every image field has a path, so with the FileSystem storage it naively tries using the empty string and we get a IsADirectoryError on the MEDIA_ROOT.

[paxapy]

i got this on each setup call in tests :c because we had signal connected to instance save method to warm images. so I've added if instance.image condition before calling .warm() method to handle it

[sadman4eg]

i got this on each setup call in tests :c because we had signal connected to instance save method to warm images. so I've added if instance.image condition before calling .warm() method to handle it

thanks, pasha!