Did you perhaps disable all attachments (or not have any attachment enabled by default)? If so, it may explain why it has no bounds set if the mesh is empty.

If this does not occur later at runtime (when in play mode) but in the Editor outside of play mode, you can setup the desired starting animation and hit the Match button next to Match Rect Transform with Mesh, as explained here.

If the bounds are correct in the Editor but get incorrect at runtime, please provide more information what steps you performed that led to this issue occurring.