Blender Git Loki

Depsgraph: suppress logs from add_relation on invalid driver variables.

Rig generators (e.g. rigify) use dummy drivers to attach extra
ID references to rigs for Append/Link. These references use
Single Property variables with empty path, which is intentionally
invalid and shouldn't produce low-level error logs from depsgraph.

The removed code would always print an error because RNAPathKey
requires a valid path to produce a non-NULL reference.