Problem Description

When `glance-manage db` is run without specifying a subcommand (such as `version`, `sync`, `expand`, `contract`, etc.), the command crashes with an unhandled `AttributeError` instead of displaying a helpful usage message.

The root cause is that the code attempts to access `CONF.command.action` without first checking if it is `None`. When no subcommand is provided, `CONF.command.action` is `None`, causing the `startswith()` method call to fail.

To Reproduce Steps to reproduce the behavior:

1. Execute `glance-manage db` without any subcommand:
   ```bash
   $ glance-manage db
   ```

2. Observe the error output:
   ```
   CRITICAL glance [-] Unhandled error: AttributeError: 'NoneType' object has no attribute 'startswith'
   ERROR glance Traceback (most recent call last):
   ERROR glance File "/usr/local/bin/glance-manage", line 10, in <module>
   ERROR glance sys.exit(main())
   ERROR glance File "/opt/stack/glance/glance/cmd/manage.py", line 558, in main
   ERROR glance if CONF.command.action.startswith('db'):
   ERROR glance AttributeError: 'NoneType' object has no attribute 'startswith'

Expected behavior

• When `glance-manage db` is executed without a subcommand, it should:

• Display a helpful message listing all available database subcommands
• Exit gracefully with an appropriate exit code (e.g., 2 for usage error)
• Not raise an unhandled exception