close sftp connection without error (#7953)
close sftp connection without error (#7953)

check self.conn is None and close connection without error like
SSHHook
dhuang/incubator-airflow,apache/airflow,DinoCow/airflow,wooga/airflow,lyft/incubator-airflow,mrkm4ntr/incubator-airflow,Acehaidrey/incubator-airflow,cfei18/incubator-airflow,apache/incubator-airflow,apache/airflow,nathanielvarona/airflow,Acehaidrey/incubator-airflow,apache/incubator-airflow,danielvdende/incubator-airflow,cfei18/incubator-airflow,dhuang/incubator-airflow,Acehaidrey/incubator-airflow,DinoCow/airflow,sekikn/incubator-airflow,apache/incubator-airflow,mistercrunch/airflow,sekikn/incubator-airflow,airbnb/airflow,DinoCow/airflow,sekikn/incubator-airflow,nathanielvarona/airflow,nathanielvarona/airflow,mrkm4ntr/incubator-airflow,cfei18/incubator-airflow,mistercrunch/airflow,danielvdende/incubator-airflow,nathanielvarona/airflow,bolkedebruin/airflow,Acehaidrey/incubator-airflow,danielvdende/incubator-airflow,cfei18/incubator-airflow,airbnb/airflow,apache/airflow,lyft/incubator-airflow,lyft/incubator-airflow,danielvdende/incubator-airflow,airbnb/airflow,cfei18/incubator-airflow,danielvdende/incubator-airflow,apache/airflow,mistercrunch/airflow,danielvdende/incubator-airflow,dhuang/incubator-airflow,mrkm4ntr/incubator-airflow,wooga/airflow,DinoCow/airflow,Acehaidrey/incubator-airflow,cfei18/incubator-airflow,bolkedebruin/airflow,nathanielvarona/airflow,mistercrunch/airflow,bolkedebruin/airflow,airbnb/airflow,sekikn/incubator-airflow,dhuang/incubator-airflow,bolkedebruin/airflow,wooga/airflow,wooga/airflow,mrkm4ntr/incubator-airflow,nathanielvarona/airflow,bolkedebruin/airflow,lyft/incubator-airflow,apache/airflow,apache/airflow,Acehaidrey/incubator-airflow,apache/incubator-airflow