Prevent 500 for anon /api/invocations

Fixes:
```
ExceptionGroup: unhandled errors in a TaskGroup
  File "starlette/_utils.py", line 87, in collapse_excgroups
    yield
  File "starlette/middleware/base.py", line 190, in __call__
    async with anyio.create_task_group() as task_group:
  File "anyio/_backends/_asyncio.py", line 678, in __aexit__
    raise BaseExceptionGroup(
AttributeError: 'NoneType' object has no attribute 'id'
  File "starlette/applications.py", line 123, in __call__
    await self.middleware_stack(scope, receive, send)
  File "starlette/middleware/errors.py", line 186, in __call__
    raise exc
  File "starlette/middleware/errors.py", line 164, in __call__
    await self.app(scope, receive, _send)
  File "starlette_context/middleware/raw_middleware.py", line 92, in __call__
    await self.app(scope, receive, send_wrapper)
  File "starlette/middleware/base.py", line 189, in __call__
    with collapse_excgroups():
  File "contextlib.py", line 155, in __exit__
    self.gen.throw(typ, value, traceback)
  File "starlette/_utils.py", line 93, in collapse_excgroups
    raise exc
  File "starlette/middleware/base.py", line 191, in __call__
    response = await self.dispatch_func(request, call_next)
  File "galaxy/webapps/galaxy/fast_app.py", line 108, in add_x_frame_options
    response = await call_next(request)
  File "starlette/middleware/base.py", line 165, in call_next
    raise app_exc
  File "starlette/middleware/base.py", line 151, in coro
    await self.app(scope, receive_or_disconnect, send_no_error)
  File "starlette/middleware/exceptions.py", line 62, in __call__
    await wrap_app_handling_exceptions(self.app, conn)(scope, receive, send)
  File "starlette/_exception_handler.py", line 64, in wrapped_app
    raise exc
  File "starlette/_exception_handler.py", line 53, in wrapped_app
    await app(scope, receive, sender)
  File "starlette/routing.py", line 758, in __call__
    await self.middleware_stack(scope, receive, send)
  File "starlette/routing.py", line 778, in app
    await route.handle(scope, receive, send)
  File "starlette/routing.py", line 299, in handle
    await self.app(scope, receive, send)
  File "starlette/routing.py", line 79, in app
    await wrap_app_handling_exceptions(app, request)(scope, receive, send)
  File "starlette/_exception_handler.py", line 64, in wrapped_app
    raise exc
  File "starlette/_exception_handler.py", line 53, in wrapped_app
    await app(scope, receive, sender)
  File "starlette/routing.py", line 74, in app
    response = await func(request)
  File "fastapi/routing.py", line 278, in app
    raw_response = await run_endpoint_function(
  File "fastapi/routing.py", line 193, in run_endpoint_function
    return await run_in_threadpool(dependant.call, **values)
  File "starlette/concurrency.py", line 42, in run_in_threadpool
    return await anyio.to_thread.run_sync(func, *args)
  File "anyio/to_thread.py", line 56, in run_sync
    return await get_async_backend().run_sync_in_worker_thread(
  File "anyio/_backends/_asyncio.py", line 2144, in run_sync_in_worker_thread
    return await future
  File "anyio/_backends/_asyncio.py", line 851, in run
    result = context.run(func, *args)
  File "galaxy/webapps/galaxy/api/workflows.py", line 1316, in index_invocations
    invocations, total_matches = self.invocations_service.index(trans, invocation_payload, serialization_params)
  File "galaxy/webapps/galaxy/services/invocations.py", line 111, in index
    user_id = trans.user.id
```
from https://sentry.galaxyproject.org/share/issue/7d75e948d2db45039544435f1c4e1eda/

lib/galaxy/webapps/galaxy/api/workflows.py

@@ -1296,6 +1296,10 @@ def index_invocations(
         include_nested_invocations: bool = True,
         trans: ProvidesUserContext = DependsOnTrans,
     ) -> List[WorkflowInvocationResponse]:
+        if not trans.user:
+            # Anon users don't have accessible invocations (currently, though published invocations should be a thing)
+            response.headers["total_matches"] = "0"
+            return []
         invocation_payload = InvocationIndexPayload(
             workflow_id=workflow_id,
             history_id=history_id,