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

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

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.

由索引模型根据 Issue 内容生成。