io.read_triangle_mesh return non-empty TriangleMesh when read an empty file with .stl extensiuon

**IMPORTANT: Please use the following template to report the bug.**
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**Describe the bug**
io.read_triangle_mesh return non-empty TriangleMesh when read an empty file with .stl extensiuon.
All the vertices are 0.0, and when I show it by `o3d.visualization.draw_geometries([mesh])`,
an exception with type of RuntimeError raised.

**To Reproduce**
Steps to reproduce the behavior:

1. create an emtpy file with stl extension: touch empty.stl
2. read created empty file by io.read_triangle_mesh:
```python
import numpy as np
import open3d.open3d as o3d

if __name__ == '__main__':
mesh = o3d.io.read_triangle_mesh('empty.stl')

assert isinstance(mesh, o3d.geometry.TriangleMesh)

print(np.asarray(mesh.vertices))
print(np.asarray(mesh.triangles))

```
3. See error

**Expected behavior**
an empty TriangleMesh should be returned, or an exception raised.

A clear and concise description of what you expected to happen.

**Screenshots**
If applicable, add screenshots to help explain your problem.

**Environment (please complete the following information):**

- Operating system: Ubuntu 16.04
- Python version: Python 3.6
- Open3D version: 0.9.0.0
- Is this remote workstation?: no
- How did you install Open3D?: pip (pip install -i https://mirrors.aliyun.com/pypi/simple/ open3d==0.9.0.0)
- Compiler version (if built from source): NO