Fix/cleanup RNA viewlayer API.

RNA's ViewLayer would present 'first level' of layer collection as a
list (collection property), when it is actually now only a single item,
same as the scene's master collection.

Note: did not try to update view_layer python tests, those are already
fully broken for quiet some time I guess (they still assume
view_layer.collections to be mutable e.g.)...

release/scripts/modules/bpy_extras/object_utils.py

@@ -121,18 +121,12 @@ def object_data_add(context, obdata, operator=None, name=None):
     """
     scene = context.scene
     layer = context.view_layer
-    layer_collection = context.layer_collection
+    layer_collection = context.layer_collection or layer.active_layer_collection
+    scene_collection = layer_collection.collection
 
     for ob in layer.objects:
         ob.select_set(action='DESELECT')
 
-    if not layer_collection:
-        # when there is no collection linked to this view_layer create one
-        scene_collection = scene.master_collection.collections.new("")
-        layer_collection = layer.collections.link(scene_collection)
-    else:
-        scene_collection = layer_collection.collection
-
     if name is None:
         name = "Object" if obdata is None else obdata.name