[OWL-1231][common][nqm-mng] Fix `GetCityById`
[OWL-1231][common][nqm-mng] Fix `GetCityById`

It returns `nil` when not found now.

masato25/open-falcon-backend,Cepave/open-falcon-backend,masato25/open-falcon-backend,Cepave/open-falcon-backend,Cepave/open-falcon-backend,Cepave/open-falcon-backend,masato25/open-falcon-backend,masato25/open-falcon-backend,Cepave/open-falcon-backend,masato25/open-falcon-backend,masato25/open-falcon-backend,Cepave/open-falcon-backend