Fix set_user function

Should fix https://sentry.galaxyproject.org/organizations/galaxy/issues/84836/?project=3&query=is%3Aunresolved&statsPeriod=14d:
```
AttributeError: 'NoneType' object has no attribute 'bootstrap_admin_user'
  File "uvicorn/protocols/http/h11_impl.py", line 366, in run_asgi
    result = await app(self.scope, self.receive, self.send)
  File "uvicorn/middleware/proxy_headers.py", line 75, in __call__
    return await self.app(scope, receive, send)
  File "fastapi/applications.py", line 269, in __call__
    await super().__call__(scope, receive, send)
  File "starlette/applications.py", line 124, in __call__
    await self.middleware_stack(scope, receive, send)
  File "starlette/middleware/errors.py", line 184, in __call__
    raise exc
  File "starlette/middleware/errors.py", line 162, in __call__
    await self.app(scope, receive, _send)
  File "starlette_context/middleware/raw_middleware.py", line 96, in __call__
    await self.app(scope, receive, send_wrapper)
  File "starlette/middleware/base.py", line 69, in __call__
    await response(scope, receive, send)
  File "starlette/responses.py", line 260, in __call__
    await wrap(partial(self.listen_for_disconnect, receive))
  File "anyio/_backends/_asyncio.py", line 662, in __aexit__
    raise exceptions[0]
  File "starlette/responses.py", line 256, in wrap
    await func()
  File "starlette/responses.py", line 245, in stream_response
    async for chunk in self.body_iterator:
  File "starlette/middleware/base.py", line 58, in body_stream
    raise app_exc
  File "starlette/middleware/base.py", line 36, in coro
    await self.app(scope, request.receive, send_stream.send)
  File "starlette/middleware/base.py", line 69, in __call__
    await response(scope, receive, send)
  File "starlette/responses.py", line 260, in __call__
    await wrap(partial(self.listen_for_disconnect, receive))
  File "anyio/_backends/_asyncio.py", line 662, in __aexit__
    raise exceptions[0]
  File "starlette/responses.py", line 256, in wrap
    await func()
  File "starlette/responses.py", line 245, in stream_response
    async for chunk in self.body_iterator:
  File "starlette/middleware/base.py", line 58, in body_stream
    raise app_exc
  File "starlette/middleware/base.py", line 36, in coro
    await self.app(scope, request.receive, send_stream.send)
  File "starlette/exceptions.py", line 93, in __call__
    raise exc
  File "starlette/exceptions.py", line 82, in __call__
    await self.app(scope, receive, sender)
  File "fastapi/middleware/asyncexitstack.py", line 21, in __call__
    raise e
  File "fastapi/middleware/asyncexitstack.py", line 18, in __call__
    await self.app(scope, receive, send)
  File "starlette/routing.py", line 670, in __call__
    await route.handle(scope, receive, send)
  File "starlette/routing.py", line 418, in handle
    await self.app(scope, receive, send)
  File "a2wsgi/wsgi.py", line 140, in __call__
    return await responder(scope, receive, send)
  File "a2wsgi/wsgi.py", line 179, in __call__
    raise self.exc_info[0].with_traceback(
  File "galaxy/web/framework/middleware/error.py", line 165, in __call__
    app_iter = self.application(environ, sr_checker)
  File "/cvmfs/main.galaxyproject.org/venv/lib/python3.8/site-packages/paste/recursive.py", line 85, in __call__
    return self.application(environ, start_response)
  File "galaxy/web/framework/middleware/statsd.py", line 29, in __call__
    req = self.application(environ, start_response)
  File "/cvmfs/main.galaxyproject.org/venv/lib/python3.8/site-packages/paste/httpexceptions.py", line 640, in __call__
    return self.application(environ, start_response)
  File "galaxy/web/framework/base.py", line 159, in __call__
    return self.handle_request(environ, start_response)
  File "galaxy/web/framework/base.py", line 215, in handle_request
    trans = self.transaction_factory(environ)
  File "galaxy/webapps/base/webapp.py", line 105, in <lambda>
    self.set_transaction_factory(lambda e: self.transaction_chooser(e, galaxy_app, session_cookie))
  File "galaxy/webapps/base/webapp.py", line 194, in transaction_chooser
    return GalaxyWebTransaction(environ, galaxy_app, self, session_cookie)
  File "galaxy/webapps/base/webapp.py", line 311, in __init__
    self.error_message = self._authenticate_api(session_cookie)
  File "galaxy/webapps/base/webapp.py", line 541, in _authenticate_api
    self.user = None
  File "galaxy/webapps/base/webapp.py", line 451, in set_user
    if user.bootstrap_admin_user:
```

Not sure what I was thinking when I did this in https://github.com/galaxyproject/galaxy/pull/10823/files#diff-ca6cf7b8ec16cb97add6d9535857de123352878e979bcc16812cdfa23f22b204R360

If we have a boostrap_admin_user we certainly can't add it to a session,
that would definitely fail.