Fix T68135: Empty curve object crashes export (STL exporter part).

Referenced by issue blender/blender#68135, Empty curve object crashes export

io_mesh_stl/blender_utils.py

@@ -87,10 +87,13 @@ def faces_from_mesh(ob, global_matrix, use_mesh_modifiers=False):
     else:
         mesh_owner = ob
 
+    # Object.to_mesh() is not guaranteed to return a mesh.
     try:
         mesh = mesh_owner.to_mesh()
     except RuntimeError:
         return
+    if mesh is None:
+        return
 
     mat = global_matrix @ ob.matrix_world
     mesh.transform(mat)