[AIRFLOW-3423] Fix mongo hook to work with anonymous access (#4258)
[AIRFLOW-3423] Fix mongo hook to work with anonymous access (#4258)

If no login/password was supplied it would generate an invalid DSN
mtagle/airflow,cfei18/incubator-airflow,bolkedebruin/airflow,Acehaidrey/incubator-airflow,DinoCow/airflow,apache/airflow,mrkm4ntr/incubator-airflow,spektom/incubator-airflow,Fokko/incubator-airflow,mrkm4ntr/incubator-airflow,Fokko/incubator-airflow,wooga/airflow,nathanielvarona/airflow,airbnb/airflow,apache/airflow,bolkedebruin/airflow,wooga/airflow,mistercrunch/airflow,mistercrunch/airflow,Acehaidrey/incubator-airflow,DinoCow/airflow,mtagle/airflow,wooga/airflow,dhuang/incubator-airflow,apache/airflow,cfei18/incubator-airflow,sekikn/incubator-airflow,nathanielvarona/airflow,airbnb/airflow,cfei18/incubator-airflow,danielvdende/incubator-airflow,mistercrunch/airflow,DinoCow/airflow,spektom/incubator-airflow,Fokko/incubator-airflow,Acehaidrey/incubator-airflow,nathanielvarona/airflow,dhuang/incubator-airflow,mrkm4ntr/incubator-airflow,mtagle/airflow,Acehaidrey/incubator-airflow,bolkedebruin/airflow,airbnb/airflow,danielvdende/incubator-airflow,wileeam/airflow,nathanielvarona/airflow,dhuang/incubator-airflow,lyft/incubator-airflow,r39132/airflow,spektom/incubator-airflow,cfei18/incubator-airflow,bolkedebruin/airflow,mistercrunch/airflow,Acehaidrey/incubator-airflow,r39132/airflow,mtagle/airflow,cfei18/incubator-airflow,r39132/airflow,danielvdende/incubator-airflow,sekikn/incubator-airflow,lyft/incubator-airflow,nathanielvarona/airflow,Acehaidrey/incubator-airflow,airbnb/airflow,dhuang/incubator-airflow,bolkedebruin/airflow,danielvdende/incubator-airflow,DinoCow/airflow,Fokko/incubator-airflow,wooga/airflow,wileeam/airflow,apache/incubator-airflow,lyft/incubator-airflow,apache/airflow,apache/incubator-airflow,lyft/incubator-airflow,sekikn/incubator-airflow,apache/airflow,apache/incubator-airflow,danielvdende/incubator-airflow,apache/airflow,apache/incubator-airflow,nathanielvarona/airflow,sekikn/incubator-airflow,wileeam/airflow,spektom/incubator-airflow,r39132/airflow,mrkm4ntr/incubator-airflow,wileeam/airflow,danielvdende/incubator-airflow,cfei18/incubator-airflow