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**Apache Airflow version**: 1.10.10

**What happened**:

I wrote the following piece of code:
```
from datetime import datetime
from airflow import DAG
from airflow.contrib.operators.ssh_operator import SSHOperator


args = {
    'owner': 'airflow',
    'start_date': datetime(year=2020, month=7, day=21,
                           hour=3, minute=0, second=0),
    'provide_context': True,
}

dag = DAG(
    dag_id='test_ssh_operator',
    default_args=args,
    schedule_interval='@daily',
)

ssh_command = """
echo 'hello work'
"""
task = SSHOperator(
    task_id="check_ssh_perator",
    ssh_conn_id='ssh_default',
    command=ssh_command,
    do_xcom_push=True,
    dag=dag,
)

task
```
<!-- (please include exact error messages if you can) -->

**What you expected to happen**:
And I got the following error:

`Broken DAG: [/etc/airflow/dags/dag_test_sshoperator.py] 'NoneType' object has no attribute 'startswith'`
<!-- What do you think went wrong? -->
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**Anything else we need to know**:
I add new ssh connection in Admin--> Connections. and in Extra field, I put the following JSON:

```
{"key_file":"/root/.ssh/airflow-connector/id_ed25519",
"timeout": "10",
"compress": "false",
"no_host_key_check": "false",
"allow_host_key_change": "false"}
```

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tests/providers/ssh/operators/test_ssh.py
with self.assertRaisesRegex(AirflowException, "Cannot operate without ssh_hook or ssh_conn_id."):
            task_0 = SSHOperator(task_id="test", command=COMMAND, timeout=TIMEOUT, dag=self.dag)
            task_0.execute(None)
        
        task_1 = SSHOperator(
            task_id="test_1",
            ssh_hook="string_rather_than_SSHHook",  
            ssh_conn_id=TEST_CONN_ID,
            command=COMMAND,
            timeout=TIMEOUT,
            dag=self.dag,
        )
        try:
            task_1.execute(None)
        except Exception:  
            pass
        self.assertEqual(task_1.ssh_hook.ssh_conn_id, TEST_CONN_ID)
        task_2 = SSHOperator(
            task_id="test_2",
            ssh_conn_id=TEST_CONN_ID,  
            command=COMMAND,
            timeout=TIMEOUT,