Blender Git Loki

Fix T84935: improve previous fix for boolean custom data layers

This improves the fix introduced in rBdc8b31af578bfcf9f77dbce6d7b7da1006a3c8c1.
While it did not result in other bugs afaik, I thought it would be
good to make the fix more specific, in case some other code did
expect `layer->data` to be null. This updated fix checks for the
very specific case the fix was intended for.