Fix: Incorrect vertex group layer "construct" callback

The "set default" callback doesn't need to be defined since it falls
back to clearing the memory, but since "construct" is optional, it
needs to be defined. Mistake in 25237d2625.

source/blender/blenkernel/intern/customdata.cc

@@ -1651,8 +1651,8 @@ static const LayerTypeInfo LAYERTYPEINFO[CD_NUMTYPES] = {
      layerFree_mdeformvert,
      layerInterp_mdeformvert,
      nullptr,
-     layerConstruct_mdeformvert,
-     nullptr},
+     nullptr,
+     layerConstruct_mdeformvert},
     /* 3: CD_MEDGE */
     {sizeof(MEdge), "MEdge", 1, nullptr, nullptr, nullptr, nullptr, nullptr, nullptr},
     /* 4: CD_MFACE */