**Apache Airflow version**: 2.0.0

**What happened**:

While investigating issues relating to task getting stuck, I saw this sql error in postgres logs. I am not entirely sure of what it impacts but I thought of letting you know.

```
ERROR:  column "connection.password" must appear in the GROUP BY clause or be used in an aggregate function at character 8
STATEMENT:  SELECT connection.password AS connection_password, connection.extra AS connection_extra, connection.id AS connection_id, connection.conn_id AS connection_conn_id, connection.conn_type AS connection_conn_type, connection.description AS connection_description, connection.host AS connection_host, connection.schema AS connection_schema, connection.login AS connection_login, connection.port AS connection_port, connection.is_encrypted AS connection_is_encrypted, connection.is_extra_encrypted AS connection_is_extra_encrypted, count(connection.conn_id) AS count_1 
        FROM connection GROUP BY connection.conn_id 
        HAVING count(connection.conn_id) > 1
ERROR:  current transaction is aborted, commands ignored until end of transaction block
STATEMENT:  SELECT connection.password AS connection_password, connection.extra AS connection_extra, connection.id AS connection_id, connection.conn_id AS connection_conn_id, connection.conn_type AS connection_conn_type, connection.description AS connection_description, connection.host AS connection_host, connection.schema AS connection_schema, connection.login AS connection_login, connection.port AS connection_port, connection.is_encrypted AS connection_is_encrypted, connection.is_extra_encrypted AS connection_is_extra_encrypted 
        FROM connection 
        WHERE connection.conn_type IS NULL
```

**How to reproduce it**:
1. Run `docker-compose run initdb`
2. Run `docker-compose run upgradedb`


<details> <summary> Here's my docker-compose </summary>

```
version: "3.2"
networks:
    airflow:

services:
    postgres:
        container_name: af_postgres
        image: postgres:9.6
        environment:
            - POSTGRES_USER=airflow
            - POSTGRES_DB=airflow
            - POSTGRES_PASSWORD=airflow
        volumes:
            - ./postgresql/data:/var/lib/postgresql/data
        command: >
            postgres
            -c listen_addresses=*
            -c logging_collector=on
            -c log_destination=stderr
        networks:
            - airflow
    initdb:
        container_name: af_initdb
        image: docker.io/apache/airflow:2.0.0-python3.7
        environment:
            - AIRFLOW__CORE__SQL_ALCHEMY_CONN=postgresql+psycopg2://airflow:airflow@postgres:5432/airflow
        depends_on:
            - postgres
        entrypoint: /bin/bash
        command: -c "airflow db init"
        networks:
            - airflow
    upgradedb:
        container_name: af_upgradedb
        image: docker.io/apache/airflow:2.0.0-python3.7
        environment:
            - AIRFLOW__CORE__SQL_ALCHEMY_CONN=postgresql+psycopg2://airflow:airflow@postgres:5432/airflow
        depends_on:
            - postgres
        entrypoint: /bin/bash
        command: -c "airflow db upgrade"
        networks:
            - airflow
 ```
</details>

**Anything else we need to know**:

Upon looking the code, I believe having `Connection.conn_id` [here](https://github.com/apache/airflow/blob/ab5f770bfcd8c690cbe4d0825896325aca0beeca/airflow/utils/db.py#L613) will resolve the sql syntax error.

airflow/utils/db.py
"""
    Check nullable conn_type column in Connection table
    :param session:  session of the sqlalchemy
    :rtype: str
    """
    n_nulls = []
    try:
        n_nulls = session.query(Connection).filter(Connection.conn_type.is_(None)).all()
    except (exc.OperationalError, exc.ProgrammingError, exc.InternalError):
        
        pass
    if n_nulls:
        return (
            'The conn_type column in the connection '
            'table must contain content.\n'
            'Make sure you don\'t have null '
            'in the conn_type column.\n'
            f'Null conn_type conn_id: {list(n_nulls)}'
        )
    return ''
@provide_session
def auto_migrations_available(session=None):