No logic to handle case where no microscope image is provided for the frontend

Start in murfey/server/api/hub.py at get_instrument_image, where the traceback shows FileResponse receiving a None image_path. Reproduce the endpoint request for an instrument without a microscope image and trace the configured value. Done means that no-image case is handled without the shown TypeError.

There is currently no logic in the Murfey server to handle a case where no microscope image has been supplied.

    Traceback (most recent call last):

      File "/venv/lib/python3.12/site-packages/uvicorn/protocols/http/httptools_impl.py", line 409, in run_asgi
        result = await app(  # type: ignore[func-returns-value]
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

      File "/venv/lib/python3.12/site-packages/uvicorn/middleware/proxy_headers.py", line 60, in __call__
        return await self.app(scope, receive, send)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

      File "/venv/lib/python3.12/site-packages/fastapi/applications.py", line 1054, in __call__
        await super().__call__(scope, receive, send)

      File "/venv/lib/python3.12/site-packages/starlette/applications.py", line 112, in __call__
        await self.middleware_stack(scope, receive, send)

      File "/venv/lib/python3.12/site-packages/starlette/middleware/errors.py", line 187, in __call__
        raise exc

      File "/venv/lib/python3.12/site-packages/starlette/middleware/errors.py", line 165, in __call__
        await self.app(scope, receive, _send)

      File "/venv/lib/python3.12/site-packages/starlette/middleware/cors.py", line 85, in __call__
        await self.app(scope, receive, send)

      File "/venv/lib/python3.12/site-packages/starlette/middleware/exceptions.py", line 62, in __call__
        await wrap_app_handling_exceptions(self.app, conn)(scope, receive, send)

      File "/venv/lib/python3.12/site-packages/starlette/_exception_handler.py", line 53, in wrapped_app
        raise exc

      File "/venv/lib/python3.12/site-packages/starlette/_exception_handler.py", line 42, in wrapped_app
        await app(scope, receive, sender)

      File "/venv/lib/python3.12/site-packages/starlette/routing.py", line 714, in __call__
        await self.middleware_stack(scope, receive, send)

      File "/venv/lib/python3.12/site-packages/starlette/routing.py", line 734, in app
        await route.handle(scope, receive, send)

      File "/venv/lib/python3.12/site-packages/starlette/routing.py", line 288, in handle
        await self.app(scope, receive, send)

      File "/venv/lib/python3.12/site-packages/starlette/routing.py", line 76, in app
        await wrap_app_handling_exceptions(app, request)(scope, receive, send)

      File "/venv/lib/python3.12/site-packages/starlette/_exception_handler.py", line 53, in wrapped_app
        raise exc

      File "/venv/lib/python3.12/site-packages/starlette/_exception_handler.py", line 42, in wrapped_app
        await app(scope, receive, sender)

      File "/venv/lib/python3.12/site-packages/starlette/routing.py", line 73, in app
        response = await f(request)
                   ^^^^^^^^^^^^^^^^

      File "/venv/lib/python3.12/site-packages/fastapi/routing.py", line 301, in app
        raw_response = await run_endpoint_function(
                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

      File "/venv/lib/python3.12/site-packages/fastapi/routing.py", line 214, in run_endpoint_function
        return await run_in_threadpool(dependant.call, **values)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

      File "/venv/lib/python3.12/site-packages/starlette/concurrency.py", line 37, in run_in_threadpool
        return await anyio.to_thread.run_sync(func)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

      File "/venv/lib/python3.12/site-packages/anyio/to_thread.py", line 56, in run_sync
        return await get_async_backend().run_sync_in_worker_thread(
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

      File "/venv/lib/python3.12/site-packages/anyio/_backends/_asyncio.py", line 2470, in run_sync_in_worker_thread
        return await future
               ^^^^^^^^^^^^

      File "/venv/lib/python3.12/site-packages/anyio/_backends/_asyncio.py", line 967, in run
        result = context.run(func, *args)
                 ^^^^^^^^^^^^^^^^^^^^^^^^

      File "/venv/lib/python3.12/site-packages/murfey/server/api/hub.py", line 36, in get_instrument_image
        return FileResponse(config[instrument_name].image_path)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

      File "/venv/lib/python3.12/site-packages/starlette/responses.py", line 313, in __init__
        media_type = guess_type(filename or path)[0] or "text/plain"
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

      File "/usr/local/lib/python3.12/mimetypes.py", line 310, in guess_type
        return _db.guess_type(url, strict)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^

      File "/usr/local/lib/python3.12/mimetypes.py", line 122, in guess_type
        url = os.fspath(url)
              ^^^^^^^^^^^^^^

    TypeError: expected str, bytes or os.PathLike object, not NoneType