Blender Git Loki

Fix T72887: Outliner crashes when after undo of some linking operation.

Outliner tree building code was not handling properly empty libraries
(i.e. Lib datablocks in our bmain which have no used actual data
anymore).

Main issue here is unclean states of indirect hierarchies of linking
involving several libraries after undo operation.

This is not a critical issue though, just annoying and untidy.