add check for mesh before loading geometry

[shimwell]

currently loading in the wrong type of mesh tally generates an error

AttributeError: 'NoneType' object has no attribute 'geometry'
Traceback:

File "/home/j/miniforge-pypy3/envs/openmc-plot/lib/python3.10/site-packages/streamlit/runtime/scriptrunner/script_runner.py", line 542, in _run_script
    exec(code, module.__dict__)
File "/home/j/openmc_plot/src/openmc_plot/pages/7_🛢️_Cylindrical_mesh_plot.py", line 11, in <module>
    main()
File "/home/j/openmc_cylindrical_mesh_plotter/src/openmc_cylindrical_mesh_plotter/app.py", line 107, in main
    geometry = statepoint.summary.geometry