Blender Git Loki

Fix T69798: Pinning empty image objects

We have no dedicated image context tab, so for now making sure we don't
end up passing its ID as the pinned one.

If we ever get one, we then will need a different solution here, like
changing the ID that owns the data to be the object, instead of the
empty image datablock.

Differential Revision: https://developer.blender.org/D6273