FBX Export: Make empty shape keys exportable

Currently all empty shape keys are not exported when exporting as FBX. The reason for this is that empty shape keys cause issues like crashing Unity. This can cause confusion because it is done without explanation.
This patch fixes this by making the shape keys technically empty while still keeping them intact and working as shape keys.

Reviewers: campbellbarton, mont29

Reviewed By: mont29

Tags: #bf_blender_2.8, #addons

Differential Revision: https://developer.blender.org/D4496

io_scene_fbx/export_fbx_bin.py

@@ -2245,7 +2245,6 @@ def fbx_data_from_scene(scene, depsgraph, settings):
 
         for shape in me.shape_keys.key_blocks[1:]:
             # Only write vertices really different from org coordinates!
-            # XXX FBX does not like empty shapes (makes Unity crash e.g.), so we have to do this here... :/
             shape_verts_co = []
             shape_verts_idx = []
 
@@ -2258,8 +2257,13 @@ def fbx_data_from_scene(scene, depsgraph, settings):
                     continue
                 shape_verts_co.extend(Vector(sv_co) - Vector(ref_co))
                 shape_verts_idx.append(idx)
+
+            # FBX does not like empty shapes (makes Unity crash e.g.).
+            # To prevent this, we add a vertex that does nothing, but it keeps the shape key intact
             if not shape_verts_co:
-                continue
+                shape_verts_co.extend((0, 0, 0))
+                shape_verts_idx.append(0)
+
             channel_key, geom_key = get_blender_mesh_shape_channel_key(me, shape)
             data = (channel_key, geom_key, shape_verts_co, shape_verts_idx)
             data_deformers_shape.setdefault(me, (me_key, shapes_key, {}))[2][shape] = data