* fix: retarget is not performed if rootBone is the only bone to be retargeted
* fix: MeshRetargeter may cause NRE if mesh is null
* fix: MeshRetargeter is not working if sharedMesh is null
* test: add test case for SkinnedMeshRenderer only with rootBone
* test: fix RootBoneOnly test
* test: fix expected and actual
* test: add test for SkinnedMeshRenderer without mesh