Fix: Bone collection renaming when empty string

Wrong default value was passed to `BLI_uniquename`.
This default value is used when string is empty after renaming.
the element.

How to reproduce:
- Add bone collection
- double click to rename
- Clear all characters, hit enter (you'll either notice bone collection
  without name or just a suffix)

Pull Request: https://projects.blender.org/blender/blender/pulls/111723

source/blender/animrig/intern/bone_collections.cc

@@ -92,7 +92,7 @@ static void bonecoll_ensure_name_unique(bArmature *armature, BoneCollection *bco
 {
   BLI_uniquename(&armature->collections,
                  bcoll,
-                 bcoll->name,
+                 "Bones",
                  '.',
                  offsetof(BoneCollection, name),
                  sizeof(bcoll->name));