Clearing custom split normals would get rid of the CD_CUSTOMLOOPNORMAL layer - but editing data lnor_spacearr would be kept.

Adding a CD_CUSTOMLOOPNORMAL layer (if none exists yet) should be done in edbm_average_normals_exec / BKE_editmesh_lnorspace_update / BM_lnorspace_update / BM_lnorspacearr_store.
The thing is that if the editing data lnor_spacearr would still be valid after Clear Custom Split Normals Data, blender would happily call BM_lnorspace_rebuild instead.
Doing that without a CD_CUSTOMLOOPNORMAL layer is asking for trouble.

Now clear lnor_spacearr on Clear Custom Split Normals Data as well.

Thx @Bastien Montagne (mont29) for feedback here.