 **Apache Airflow version**: 2.0.0b3


**Kubernetes version (if you are using kubernetes)** (use `kubectl version`):

**Environment**:

- **Cloud provider or hardware configuration**:
- **OS** (e.g. from /etc/os-release): Ubuntu 20.04.1 LTS 
- **Kernel** (e.g. `uname -a`): Linux airflowvm 5.4.0-56-generic #62-Ubuntu SMP Mon Nov 23 19:20:19 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux
- **Install tools**:
- **Others**:

**What happened**:

After connecting to SQLite with `airflow db shell`
And exiting the shell with`.quit`

I got the following error:
```
[2020-12-04 07:31:28,506] {process_utils.py:149} INFO - Executing cmd: sqlite3 /home/airflow/airflow/airflow.db
SQLite version 3.31.1 2020-01-27 19:55:54
Enter ".help" for usage hints.
sqlite> ;
sqlite> .quit
Traceback (most recent call last):
  File "/home/airflow/sandbox/bin/airflow", line 8, in <module>
    sys.exit(main())
  File "/home/airflow/sandbox/lib/python3.8/site-packages/airflow/__main__.py", line 40, in main
    args.func(args)
  File "/home/airflow/sandbox/lib/python3.8/site-packages/airflow/cli/cli_parser.py", line 50, in command
    return func(*args, **kwargs)
  File "/home/airflow/sandbox/lib/python3.8/site-packages/airflow/utils/cli.py", line 86, in wrapper
    return f(*args, **kwargs)
  File "/home/airflow/sandbox/lib/python3.8/site-packages/airflow/cli/commands/db_command.py", line 78, in shell
    execute_interactive(["sqlite3", url.database]).wait()
AttributeError: 'NoneType' object has no attribute 'wait'
```

**What you expected to happen**:

No error when exiting the session.
Looks like this `execute_interactive(["sqlite3", url.database])` returns `None`

**How to reproduce it**:

```
airflow db shell
sqlite> .quit
```

**Anything else we need to know**:

I love this new command :)
airflow/cli/commands/db_command.py
#
#
"""Database sub-commands"""
import os
import textwrap
from tempfile import NamedTemporaryFile
from airflow import settings
from airflow.exceptions import AirflowException
from airflow.utils import cli as cli_utils, db
from airflow.utils.process_utils import execute_interactive
def initdb(args):