[base] Minor API improvement for default variation axis setting.

* src/base/ftmm.c (FT_Set_MM_Design_Coordinates,
FT_Set_Var_Design_Coordinates, FT_Set_MM_Blend_Coordinates,
FT_Set_Var_Blend_Coordinates): Allow coords==NULL if num_coords==0.

* docs/CHANGES: Updated.
4 files changed