fix mesh 'NoneType' if confflow empty

[FaserF]

After updating to V0.2.0 I noticed the following error:

Device [AX3600] async_update_data error: 'NoneType' object is not subscriptable
Traceback (most recent call last):
  File "/config/custom_components/openwrt/coordinator.py", line 330, in async_update_data
    result['mesh'] = await self.update_mesh(wireless_config['mesh'])
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/config/custom_components/openwrt/coordinator.py", line 105, in update_mesh
    for mac in self.find_mesh_peers(conf['mesh_id']):
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/config/custom_components/openwrt/coordinator.py", line 77, in find_mesh_peers
    for _, mesh in data['mesh'].items():
                   ~~~~^^^^^^^^
TypeError: 'NoneType' object is not subscriptable

This PR fixes this. The reason is that I left mesh empty at config flow, but I am using mesh.

[kvj]

Thanks, bumped version to 0.2.1