Fix empty construction of ModelEngineOutput when exception is raised

Fix #272 (closed)

When one of the following exceptions is raised VertexModelInvalidArgument, VertexModelInternalError during a /v2/completion call, this error happens:

Traceback (most recent call last):
  File "/opt/venv/codesuggestions-9TtSrW0h-py3.9/lib/python3.9/site-packages/anyio/streams/memory.py", line 98, in receive
    return self.receive_nowait()
  File "/opt/venv/codesuggestions-9TtSrW0h-py3.9/lib/python3.9/site-packages/anyio/streams/memory.py", line 93, in receive_nowait
    raise WouldBlock
anyio.WouldBlock

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/opt/venv/codesuggestions-9TtSrW0h-py3.9/lib/python3.9/site-packages/starlette/middleware/base.py", line 43, in call_next
    message = await recv_stream.receive()
  File "/opt/venv/codesuggestions-9TtSrW0h-py3.9/lib/python3.9/site-packages/anyio/streams/memory.py", line 118, in receive
    raise EndOfStream
anyio.EndOfStream

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/app/codesuggestions/api/middleware.py", line 107, in dispatch
    response = await call_next(request)
  File "/opt/venv/codesuggestions-9TtSrW0h-py3.9/lib/python3.9/site-packages/starlette/middleware/base.py", line 46, in call_next
    raise app_exc
  File "/opt/venv/codesuggestions-9TtSrW0h-py3.9/lib/python3.9/site-packages/starlette/middleware/base.py", line 36, in coro
    await self.app(scope, request.receive, send_stream.send)
  File "/opt/venv/codesuggestions-9TtSrW0h-py3.9/lib/python3.9/site-packages/starlette/middleware/authentication.py", line 48, in __call__
    await self.app(scope, receive, send)
  File "/opt/venv/codesuggestions-9TtSrW0h-py3.9/lib/python3.9/site-packages/starlette/middleware/base.py", line 68, in __call__
    response = await self.dispatch_func(request, call_next)
  File "/app/codesuggestions/api/middleware.py", line 277, in dispatch
    return await call_next(request)
  File "/opt/venv/codesuggestions-9TtSrW0h-py3.9/lib/python3.9/site-packages/starlette/middleware/base.py", line 46, in call_next
    raise app_exc
  File "/opt/venv/codesuggestions-9TtSrW0h-py3.9/lib/python3.9/site-packages/starlette/middleware/base.py", line 36, in coro
    await self.app(scope, request.receive, send_stream.send)
  File "/opt/venv/codesuggestions-9TtSrW0h-py3.9/lib/python3.9/site-packages/starlette/exceptions.py", line 93, in __call__
    raise exc
  File "/opt/venv/codesuggestions-9TtSrW0h-py3.9/lib/python3.9/site-packages/starlette/exceptions.py", line 82, in __call__
    await self.app(scope, receive, sender)
  File "/opt/venv/codesuggestions-9TtSrW0h-py3.9/lib/python3.9/site-packages/fastapi/middleware/asyncexitstack.py", line 21, in __call__
    raise e
  File "/opt/venv/codesuggestions-9TtSrW0h-py3.9/lib/python3.9/site-packages/fastapi/middleware/asyncexitstack.py", line 18, in __call__
    await self.app(scope, receive, send)
  File "/opt/venv/codesuggestions-9TtSrW0h-py3.9/lib/python3.9/site-packages/starlette/routing.py", line 670, in __call__
    await route.handle(scope, receive, send)
  File "/opt/venv/codesuggestions-9TtSrW0h-py3.9/lib/python3.9/site-packages/starlette/routing.py", line 266, in handle
    await self.app(scope, receive, send)
  File "/opt/venv/codesuggestions-9TtSrW0h-py3.9/lib/python3.9/site-packages/starlette/routing.py", line 65, in app
    response = await func(request)
  File "/opt/venv/codesuggestions-9TtSrW0h-py3.9/lib/python3.9/site-packages/fastapi/routing.py", line 231, in app
    raw_response = await run_endpoint_function(
  File "/opt/venv/codesuggestions-9TtSrW0h-py3.9/lib/python3.9/site-packages/fastapi/routing.py", line 160, in run_endpoint_function
    return await dependant.call(**values)
  File "/opt/venv/codesuggestions-9TtSrW0h-py3.9/lib/python3.9/site-packages/dependency_injector/wiring.py", line 994, in _patched
    return await _async_inject(
  File "src/dependency_injector/_cwiring.pyx", line 66, in _async_inject
  File "/app/codesuggestions/api/v2/endpoints/code.py", line 99, in completions
    experiments=suggestion.metadata.experiments,
AttributeError: 'NoneType' object has no attribute 'experiments'